Is it in line with the pali cannon suttas to accept other scriptures?

Are human beings capable of discerning what dhamma is outside of the suttas?

Yes, when one understand the path enough.

Is it possible to know enough from the suttas and then make use of the Abhidhamma?

I make use of Abhidhamma for whole my life. I am a happiest man in the world.

What about the mere possibility that a dhamma practitioner could make use of Jesus's teachings, Hindu teachings, Sufi teachings or sutras outside the the official Theravada context?

They are included in Tipitaka already, so we call the Buddha "Sabbaññū".

When is it appropriate to be more closed minded?

When one doesn't recite, memorize, practice and understand Suttas enough to keep their mind in the middle path forever.

When is it appropriate to be more open minded?

Vinaya: When one recite, memorize, practice and understand Suttas enough to keep their mind in the middle path forever.

Commentary: When one has the quality enough. This is the qualification list.

Are human beings capable of discerning what dhamma is outside of the suttas?

Clearly they are, since otherwise the suttas could not have been written.

Is it possible to know enough from the suttas and then make use of the Abhidhamma?

Odd question. I'm not sure it would be strictly necessary to know the suttas to make use of the Abhidhamma.

What about the mere possibility that a dhamma practitioner could make use of Jesus's teachings, Hindu teachings, Sufi teachings or sutras outside the the official Theravada context?

I'd say go for it. The idea that the Buddha's teachings are unique in their message is about as daft as the idea that those of Jesus are unique. The whole point of the suttas is to tell us we can discover the truth for ourselves, which means that the Buddha does not have monopoly on knowledge. The idea that what the Buddha taught is somehow at odds with what the rest of the mystic teachers teach is profoundly damaging to Buddhism.

My feeling is that Theravadins feel the need for the Buddha to be uniquely authoritative and for all other teachers to be untrustworthy but Mahayanists do not. By endorsing Nagarjuna the latter become part of the 'Perennial' philosophy and lines itself up up with Sufism, the Upanishads and the gnostic teachings of Jesus.

When is it appropriate to be more closed minded? When is it appropriate to be more open minded?

I don't think it is about being closed or open minded. It is about doing the research and using ones powers of discrimination and reason.

Actually, even by going with the suttas, that's not enough. The Buddha was more of a hands-on kind of teacher, and He always advocate one to put the money where the mouth is and hold off on jumping to conclusion on any teaching, be it Hindu, Sufi, Christian,... or even Buddhist. And say if 2 guys were brought in front of the Buddha: one was a "Buddhist" who commits killing, stealing, adultery, lying, taking intoxicants, while the other was a follower of an "outside" sect but abstains from all of those unwholesome conducts. Guess who'd be considered a better "Buddhist" in the Buddha's eye?

"Gotami, the qualities of which you may know, 'These qualities lead to passion, not to dispassion; to being fettered, not to being unfettered; to accumulating, not to shedding; to self-aggrandizement, not to modesty; to discontent, not to contentment; to entanglement, not to seclusion; to laziness, not to aroused persistence; to being burdensome, not to being unburdensome': You may categorically hold, 'This is not the Dhamma, this is not the Vinaya, this is not the Teacher's instruction.'

"As for the qualities of which you may know, 'These qualities lead to dispassion, not to passion; to being unfettered, not to being fettered; to shedding, not to accumulating; to modesty, not to self-aggrandizement; to contentment, not to discontent; to seclusion, not to entanglement; to aroused persistence, not to laziness; to being unburdensome, not to being burdensome': You may categorically hold, 'This is the Dhamma, this is the Vinaya, this is the Teacher's instruction.'" ~~ AN 8.53 ~~

It is a wide spread idea that ''all religions'' talk about the same thing, or that all religions are more or less compatible or that the dhamma is compatible with some doctrine invented by some puthujjanas. For instance some people say that Jesus was an arahant, or an Anagamin. Some people say that because the buddha does not say explicitly that there is no god, judaism, christianity and so on are not false,

And then outside explicit religion, some puthujjanas try to build their societies on the dhamma (which is weird since samma sankappa is the opposite of the goal of a society, which is to fulfill their cravings and ideas for comfort and work and duties, responsibilities, honor, praise, material rewards, medals, and so on) like the humanists keep saying that the dhamma is compatible with liberalism, libertarianism, socialism, communism and the idea that humans are happy when they have a salary and buy the goods they crave, when they ''have meaning'', ''a sense of purpose'' and that when the desires of all the humans are fulfilled, humans will be nice to each other and live in harmony, like the christian says when they talk about their heaven.

This is a cute idea but does not go well with the dhamma. So any ''explanation'' which is the dhamma is this one

"I will teach you the penetrative explanation that is a Dhamma explanation. Listen & pay close attention. I will speak."

"As you say, lord," the monks responded.

The Blessed One said: "And which penetrative explanation is a Dhamma explanation?

"Sensuality should be known. The cause by which sensuality comes into play should be known. The diversity in sensuality should be known. The result of sensuality should be known. The cessation of sensuality should be known. The path of practice for the cessation of sensuality should be known.

"Feeling should be known. The cause by which feeling comes into play should be known. The diversity in feeling should be known. The result of feeling should be known. The cessation of feeling should be known. The path of practice for the cessation of feeling should be known.

"Perception should be known. The cause by which perception comes into play should be known. The diversity in perception should be known. The result of perception should be known. The cessation of perception should be known. The path of practice for the cessation of perception should be known.

"Fermentations[1] should be known. The cause by which fermentations come into play should be known. The diversity in fermentations should be known. The result of fermentations should be known. The cessation of fermentations should be known. The path of practice for the cessation of fermentations should be known

"Kamma should be known. The cause by which kamma comes into play should be known. The diversity in kamma should be known. The result of kamma should be known. The cessation of kamma should be known. The path of practice for the cessation of kamma should be known.

"Stress should be known. The cause by which stress comes into play should be known. The diversity in stress should be known. The result of stress should be known. The cessation of stress should be known. The path of practice for the cessation of stress should be known.

https://www.accesstoinsight.org/tipitaka/an/an06/an06.063.than.html

as long as some poeple claim to ''explain'' something and it is exactly what is mentioned in the sutta, you can say the explanation is the dhamma. But is the explanation is different or less or more than this, then it is not the dhamma.

What is true is that plenty of the wrong views created by puthujjanas allow them to do meritorious activities and they have plenty of deva realms to go to. You do not need to cultivate right view, even less cultivate wisdom to do activities with good karma. The buddha even says that the non returners cannot even go to the highest realms, only puhutjjanas can die and be reborn here as some devas. just like only puthujjanas can go to bad realm (because non-puthujjanas are said to not do activities from bad karma (because those people do not have bad intention)).

you can cultivate metta and dana, and some weird samadhi created by puthujjanas, and those are still meritorious activities, but as long as they are not seen from wisdom, right view, right investigation, sati and whatever the buddha praises, it will not lead to nibanna.

Other teachers do not teach all four types of clinging;

1. Clinging to sensuality
2. Clinging to rites and rituals
3. Clinging to views
4. Clinging to a doctrine of self

https://www.accesstoinsight.org/lib/authors/nanamoli/wheel390.html

Only Buddhadhamma is for the abandonment of all four. Other teachings in as far as they include abandonment of a type of clinging are to that extent wise.

Brahmajala sutta explains various wrong views and speculative doctrines about the world which are basis for other religions.

Also when hearing or reading teachings it should be compared to teachings known to be true, to see if they align and if they do they align they then ought to be accepted. I think this is in the great references sutta.

Making use of Abhidhamma depends on what you want with it. It's quite useful for improving expression and understanding of the Dhamma as it breaks down many teachings word for word, it will improve understanding of terms like jhana, factors of enlightenment etc. In this sense it is very useful imo.

Are human beings capable of discerning what dhamma is outside of the suttas?

That's a mark of a Sotapanna in this sense; In the Sarakaani Sutta Buddha says that if trees could discern what is rightly spoken he would proclaim them a Sotapanna how much more so his son Sarakaani who took to drink https://www.accesstoinsight.org/tipitaka/sn/sn55/sn55.024.wlsh.html