Are there differences in the way Buddhism and Theism treats scripture?

Buddha's message is difficult to understand and most people need training for understanding to mature.

Even at the time of the Buddha we read about monks who held wrong views.

Eventually the monastic community split into many pieces on account of the interpretative differences and schools produced commentary & some even a new sets of discourses attributed to the Buddha.

On the surface the difference between schools are primarily as to which texts are considered canonical & true and the interpretations of said texts are a matter of commentary on which there can be disagreement.

The scrpture is the final authority and there is a general agreement on what texts are certainly true but there are always people who will have their views contradicted by certain texts and will try to discredit those texts as somehow corrupted in order to dismiss those.

I think that "buddhavacana" are revered, and studied, maybe in the same kind of way as "Theistic" texts.

For for example does Buddhism treat its scripture as the final Authority on the nature of reality?

Yes and no (maybe "no" to the extent that scripture is just words, and words are empty!).

I think this is a good summary of the Buddhist attitude to doctrine:

Dhammānussati The Dhamma is well declared by the Bhagavā: visible here and now, immediate, inviting to come and see, effective, to be individually ascertained by the wise.

Is something considered true just for the reason that it is said in Scripture?

Well I consider it "true" when it accurately describes my experience.

Can scripture be argued with?

Well "argument" might be disparaged as counter-productive -- see for example the parable of the Blind men and the elephant in Udana 6.4 -- and I think that suttas warn that arguments are based on conceit.

Not that arguments don't happen, perhaps it's because they may happen a lot that we're warned against.

But I think, hope, that scripture can be discussed.

Is it, in some sense, eternal?

In some sense, yes. One interpretation is that the Buddha "rediscovered" the dhamma -- that's it's an ancient law (of nature), discovered by a Buddha, practiced, gradually forgotten, eventually rediscovered by the next Buddha, and so on.

Possibly one difference from "Theistic" scripture is I think that theistic religions are dogmatic and require their "believers" to believe a lot of dogma. In catholicism for example there's a Credo and so far as I know if I denied any of this I might be a heretic.

In contrast, this statement from Wikipedia:

Followers in Vietnam practice differing traditions without any problem or sense of contradiction.[34] Few Vietnamese Buddhists would identify themselves as a particular kind of Buddhism, as a Christian might identify themself by a denomination, for example.

See also these three comments from users of this site, talking about different perspectives or "schools" of Buddhism.

Personally I find there are elements of Buddhist scrpture that I wouldn't want to discard or deny -- because they seem true and useful, self-evident, beneficial, and a "refuge".

There are other elements of scripture which I may understand less well. My attitude such as it is towards those elements might be to "neither believe nor disbelieve".

Just to be thorough and clear, every faith has mystical sects, liturgical sects, and most gradations between. Mystical sects tend to treat scripture as a blueprint for spiritual attainment; liturgical sects tend to treat scripture as rock-solid revelation. Buddhism is no different. If we go to southeast Asia (or even certain communities in the US or Europe) we can find groups that take buddhist scripture as an object of reverence rather than an object of study and understanding; who light incense and ring bells and chant the names of Buddha to seek his blessing or intercession.

In the West we mainly see more mystical, philosophical forms of Buddhism. Buddhism is new to the West — it only started making inroads in the late 19th and early 20th centuries — so it hasn't had enough time for the kind of acculturation that's essential for forming liturgical communities. Western Buddhists are mainly drawn from people dissatisfied with Christian liturgical traditions; people who are interested in a deeper, more philosophical, more mystical approaches to faith, yet shy away from Christian mysticism (gnosticism, transcendentalism, Quakerism...). That should resolve itself in time.

The Buddha advocates active investigation and practice (vs. blind faith):

MN80:16.5: Let a sensible person come—neither devious nor deceitful, a person of integrity. I teach and instruct them.
MN80:16.6: Practicing as instructed they will soon know and see for themselves,

The Buddha focuses on the Four Noble Truths regarding suffering (v.s. the nature of reality).

MN141:8.3: What four?
MN141:9.1: The noble truths of suffering, the origin of suffering, the cessation of suffering, and the practice that leads to the cessation of suffering.

The Buddha advocates intelligent and informed discussion (v.s. just considering it true):

AN2.47:2.4: And when they’ve learned those teachings they question and examine each other, saying:
AN2.47:2.5: ‘Why does it say this? What does that mean?’
AN2.47:2.6: So they clarify what is unclear, reveal what is obscure, and dispel doubt regarding the many doubtful matters.
AN2.47:2.7: This is called an assembly educated in questioning, not in fancy talk.

In theistic religions, scriptures are either a form of primary revelation - an incorruptible verbatim record of the words of god(s) conveyed by prophets, or a form of secondary revelation - a divinely revealed vision seen by prophetic seers that is conveyed in their own words.

In Buddhism, the scripture mainly contains the discourses and teachings of the Buddha and his chief disciples, as recorded and passed down orally through the monastic order. The teachings of the Buddha are called the Dhamma. It is what he has left behind for his followers to guide them.

The Buddha is seen as the fully enlightened teacher, with an extraordinary ability to teach. He is not viewed as a god or an incarnation of a god. After his passing away, he is not reachable through prayers.

Thus spoke the Venerable Ananda, but the Blessed One answered him, saying: "What more does the community of bhikkhus expect from me, Ananda? I have set forth the Dhamma without making any distinction of esoteric and exoteric doctrine; there is nothing, Ananda, with regard to the teachings that the Tathagata (Buddha) holds to the last with the closed fist of a teacher who keeps some things back. ...

"Therefore, Ananda, be islands unto yourselves, refuges unto yourselves, seeking no external refuge; with the Dhamma as your island, the Dhamma as your refuge, seeking no other refuge.
DN 16

In Buddhism, the meaning is more important than the phrasing. In fact, to aid memorization of the Pali suttas, similar phrases or verses conveying the same meaning have been altered to become exactly the same, as explained in this answer.

Now, you might think, ‘These two venerables agree on the meaning but disagree on the phrasing.’ So you should approach whichever mendicant you think is most amenable and say to them: ‘The venerables agree on the meaning but disagree on the phrasing. But the venerables should know that this is how such agreement on the meaning and disagreement on the phrasing comes to be. But the phrasing is a minor matter. Please don’t get into a fight about something so minor.’
MN 103

In theistic religions, one is generally expected to have faith and unquestioningly accept the words of god(s) to be true.

In Buddhism, it's ok to question the scriptures.

"Now, Kalamas, don't go by reports, by legends, by traditions, by scripture, by logical conjecture, by inference, by analogies, by agreement through pondering views, by probability, or by the thought, 'This contemplative is our teacher.' When you know for yourselves that, 'These qualities are skillful; these qualities are blameless; these qualities are praised by the wise; these qualities, when adopted & carried out, lead to welfare & to happiness' — then you should enter & remain in them.
Kalama Sutta (AN 3.65)

In Buddhism, the Buddha simply rediscovered the wisdom that was known to previously enlightened ones before him, but have since been forgotten.

Those who were Arahants, Rightly Self-awakened Ones in the past — they, too, dwelled in dependence on the very Dhamma itself, honoring and respecting it. Those who will be Arahants, Rightly Self-awakened Ones in the future — they, too, will dwell in dependence on the very Dhamma itself, honoring and respecting it. And let the Blessed One, who is at present the Arahant, the Rightly Self-awakened One, dwell in dependence on the very Dhamma itself, honoring and respecting it."
SN 6.2

In theistic religions, it's usually not acceptable to consider other religions to be equally true, even if they teach the exact same thing. This is because the identity of the god(s) is more important than the teachings.

In Buddhism, other religions are acceptable to be true if they teach the exact same thing as Buddhism. This is because the teachings are more important than the identity of the teacher.

And the Blessed One spoke, saying: "In whatsoever Dhamma and Discipline, Subhadda, there is not found the Noble Eightfold Path, neither is there found a true ascetic of the first, second, third, or fourth degree of saintliness. But in whatsoever Dhamma and Discipline there is found the Noble Eightfold Path, there is found a true ascetic of the first, second, third, and fourth degrees of saintliness. Now in this Dhamma and Discipline, Subhadda, is found the Noble Eightfold Path; and in it alone are also found true ascetics of the first, second, third, and fourth degrees of saintliness. Devoid of true ascetics are the systems of other teachers. But if, Subhadda, the bhikkhus live righteously, the world will not be destitute of arahats.
DN 16

With regards to the universal truth of the laws of nature, we find the following statement about the three marks of existence.

“Mendicants, whether Realized Ones arise or not, this law of nature persists, this regularity of natural principles, this invariance of natural principles: all conditions are impermanent. A Realized One understands this and comprehends it, then he explains, teaches, asserts, establishes, clarifies, analyzes, and reveals it: ‘All conditions are impermanent.’

Whether Realized Ones arise or not, this law of nature persists, this regularity of natural principles, this invariance of natural principles: all conditions are suffering. A Realized One understands this and comprehends it, then he explains, teaches, asserts, establishes, clarifies, analyzes, and reveals it: ‘All conditions are suffering.’

Whether Realized Ones arise or not, this law of nature persists, this regularity of natural principles, this invariance of natural principles: all things are not-self. A Realized One understands this and comprehends it, then he explains, teaches, asserts, establishes, clarifies, analyzes, and reveals it: ‘All things are not-self.’”
AN 3.136