See my reply above regarding the second paragraph - it's a good question.

Regarding the final paragraph and implicit assumptions regarding the veracity of the content of the four canonical gospel accounts, I am not sure attributing it to "simple" assumption (i.e., the colloquial assumption where one just thinks something is true without examination) is reasonable given the education of both men and the body of work done over several centuries on examining epistemologically and historically the veracity of the gospels. See for example the intense work of John Henry Newman, 19th century Oxford scholar and intense defender of the historical reliability of the canonical gospel narratives.

Just be careful appealing to the imprecise, several century long corpus of work on the subject. By appealing to the works of the devout, people are apt to raise the equally hasty generalization described by Upton Sinclair:

“It is difficult to get a man to understand something, when his salary depends on his not understanding it.”

It seems to me that the most useful definition of "lie" is one where it does not include un-truths due to diminished faculty or simply being mistaken. I would not call Newton a "liar" because he wasn't aware of relativity...

Indeed. But one can't have it both ways. You either use your definition and call out ignorance/mistaken explicitly or you define lying as covering all unthruths.

Fulton Sheen cheats by pretending to use your definition but subsuming more than deliberately lying under the liar banner. Very sneak these philosophers.

You may be operating on a deeper level than me here, but it seems like that Fulton Sheen quote just claims that being mistaken or ignorant about whether or not you are God makes you "a lunatic". Whether you use that terminology or just replace it with "incorrect about stuff", it still seems distinct from being "a liar".

(I think we're in agreement that being mistaken about things is not incompatible with being a "great teacher", as I say in another comment here.)

Lying requires the intent to deceive. "Diminished faculty" or a disorder of mental faculties would fall under the normal definition of insanity, whatever the cause of that insanity. The claims Jesus made are too radical and too consistent to be described as mere mistakes.

Well, no, most of his claims were quite common in the time, not much is radical. Also most of those we only know 2nd or third hand and we know it's been heavily redacted on the way. If radical claims are a reason to consider someone divine, Buddha has much better cards.

The claims in question are those that are radical, specifically those involving claims of divinity, and thus not claims, if true, that can be known through unaided reason alone (i.e., the subject of divine revelation). That these claims are known through second or third hand accounts is neither here nor there. The Jesus that we know is the Jesus in those accounts so unless there's another reliable source you're leaning on (there isn't), there's little sense in talking about the "real Jesus" outside of the gospels (the most charitable reading of your use of the word "redacted" I can muster is that you mean the non-canonical gospels that were omitted from the bible, but again, those are irrelevant for reasons given elsewhere). Buddha did not make claims about being God/the son of God (which is to be expected given the Buddhist attitude toward theism) and thus the comparison in this regard is meaningless.