Is there an absolute and known hierarchy in the Dharma?

"Are there resolved authorities in what is called Buddhism or is there an array of separate, respected lineages practicing different things, believing different premises, and therefore in dispute on innumerable points of knowledge and aim?"

Before Gautama Buddha died, he oriented his disciples to look at the teachings (say, instead of looking for a person) and refused to appoint any kind of successor or "leader of the group". That is the "Buddhism" side. The social and historical side shows (many centuries afterwards) different groups with different important figures emerging, leaders of some sort of that particular group or just influential personalities or teachers.

"Are monks and nuns more advanced than what are called lay practitioners?"

Advanced in practice? Not necessarily. The monastic idea in Buddhism is for those who want to have a greater commitment, whereas dedication with lay life would not be possible or much harder. And since people have different degrees of understanding, it's natural that being monastic or lay is not, in itself, a mark of greater or lower spiritual accomplishment.

IIRC, reports of advanced lay practitioners go back to the time of the Buddha.

However, greater commitment and proper conditions means progress is expected to be faster -- again, that's the whole point of becoming monastic, to be dedicated full time to it.

"Can we assess someone's 'Buddhist progress' by how they look, the way that they dress, speak, and behave?"

Yes. Observable signs include basic traits such as honesty, kindness and harmlessness which are easier to see ordinarily. The greater the progress, the more subtle (and difficult to assess) the signs can be.

"Will someone with knowledge of the Dharma necessarily have read a large number of sermons, tracts, or scriptures from known and agreed authorities"

Not necessarily. Someone with full knowledge of the Dharma necessarily have perfect understanding of suffering, can directly see it's origin, it's ending and, having mastered the way to end it, ended it for good. Since, in the Buddha's dispensation, this is synonym with "Dharma".

That, of course, if we understanding "Dharma" above as the principles of reality of interest. Since "Dharma" can also mean "the teachings of the Buddha" (conflated from "buddha-dharma"), in that case, one certainly has to know the recorded words of the Buddha, which are his teachings.

"Are there 'solitary saints' whose innate knowledge of the Dharma is so forged as part of their being that exposure to the religion which is Buddhism was unnecessary for their role as a catalyst in waking people up?"

Generally, in the Buddhist tradition, these people are known as Buddhas: those who discovered the way out of suffering without being exposed to the teachings of another Buddha. In the tradition, all Buddhas discover the same laws, so what they teach is essentially the same.

The designations get a bit more detailed to distinguish between different people.

For example, Gautama Buddha is referred to as a sammasambuddha: a perfectly enlightened person. This designation is reserved to those who (re)discover the dharma and decide to teach. Because of their special dedication and strivings (and mistakes along the way), they are broadly developed and have full command of the "do's and don'ts" to achieve the goal -- hence being regarded as the "best teacher".

It's generally believed that no sammasambuddha arises while the teachings of a previous Buddha is known. Only when there's no trace of a previous dharma, there are conditions for someone to become a Buddha.

Another designation which is related to the question is the pratyekabuddha, referring to those Buddhas who, after (re)discovering the nature of suffering and ending it, choose to live in solitude instead of teaching.