I'll stick with the rather plain discussion of Maximus, John, etc. I just wanted to know what your thought was on the matter.
First of all, I think that this is not "plain", but rather the most difficult aspect of case of Honorius. The most plain are some other aspects, for example: was his letter an ex cathedra teaching? I think that it is impossible to deny, that his letter was an ex cathedra teaching, because Honorius was aware from the letter of Sergius, that Church union, based on monothelite doctrine, was already concluded. So Honorius was aware that whatever he had replied, his reply would had affected many people. Only in case if he had replied: "it's a difficult question, I need at first to consult with Sophronius, or to convoke a Council" — only then his reply wouldn't had been an ex cathedra teaching. And yet Catholic Encyclopedia in article on Honorius
says that Hefele and Pennacchi erred when they wrote that Honorius' letter was an ex cathedra teaching (at the end of "Modern controversies on the subject" paragraph: "These views of Hefele's ... have had a surprisingly wide influence, and have been adopted by many Catholic writers, save only his mistaken notion that a letter like that of Honorius can be supposed to fulfil the conditions laid down by the Vatican Council for an ex cathedra judgment").
Concerning more difficult aspect, with his orthodoxy — so what is, then, your opinion on Honorius' orthodoxy? From explanations of John IV and St. Maximus it follows that he was innocent, because he just replied to what Sergius asked him, and heretics intentionally misinterpreted Honorius' letter.
But then it follows that Sixth Ecumenical Council erred when at the end of the thirteenth session the Council decreed to burn his letters as "hurtful to the soul", and also at the beginning of the same session the Council decided that his first letter (his second letter was read during the same thirteenth session) was "hurtful to the soul", "quite foreign to the apostolic doctrines, and to the declarations of the holy Councils and all the Fathers of note, and follow the false doctrines of heretics.": Hefele, pp. 182-183,
paragraphs (1) and (2).
Do you agree with this consequence? This, by the way, is opinion of Pennacchi ( Hefele, p. 188
: "the Fathers of the Council had erroneously
regarded the letters of Honorius as heretical"; "He (Pennacchi) maintains ... that they (the letters of Pope Honorius) are thoroughly orthodox ... he (Honorius) meant only the will of the uncorrupted human nature of Christ (as Pope John IV. asserted, p. 52)". )
First, let me say that I have never really understood the arguments that Honorius's communications rose to an ex cathedra level. They don't really look like anything else that is debated as being in that category.
Second, Hefele can say what he wants, but when guys like John IV, Maximus the Confessor, Leo II, St. Agatho, etc. all say the guy was orthodox, I have a very tough time siding against them. If you're going to rely on the language of the Council, then you at least have to work in the same framework as them, which is Pope Agatho's epistle. Check the language in that regarding the Apostolic See. Was there any challenge to the claims he made?
As far as the letters as being hurtful to the soul, Leo II (and probably Agatho as well) would agree with that while simultaneously supporting Honorius's personal orthodoxy. Leo went so far as to say that Honorius had nourished the spread of heresy but he never called him a heretic.
This view preserves both the conciliar and papal teaching on the matter and is in accord with the contemporary witnesses who all declare for the Catholic viewpoint. This is inconvenient for Hefele, but given the accord of all the authorities here, it is the most likely conclusion.