What "historical" implies
This debate is important in distinguishing genuine historical research, based on evidence and sources, from speculation or inquiry within a historical context. Such speculation may well prove valid and it is certainly true that speculation within the historical context may come first and lead a researcher to look for the evidence to back the idea up, just as a scientists may intuit breakthroughs before they can provide the necessary formulae or proofs. There is always a danger here of confirmation bias, seeking only the evidence that fits or taking only what fits from the evidence, and this is particularly where proper method helps ground investigation.
The important thing is to distinguish what can be supported by means of primary evidence and what cannot. Just because something deals with the past and with history does not make it historical in this more academic sense.
To return specifically to two of Yggdrasilian's original points--
1.
The gap between the occultist and the serious historian is unbridgeable, because anyone committed to preserving an esoteric system of hidden knowledge will have avoided the explicit documentation of any such tradition into which one had been initiated. Thus, the documentary evidence upon which the historian relies would, in theory, always remain elusive.
Yes, the evidential methodology of history faces real and special problems when facing the world of secret traditions. In order to study or trace Hermetic history it may be necessary to extrapolate a little more than would normally be desirable because of this. For the readers of such history, speculation by someone familiar with the period, material and facts is probably going to be of more value than what they could come up with themselves, but it needs to be qualified as such.
Similar problems are seen with many aspects of "private life" in the past, from the day-to-day ideas of people to the real reasons behind many events. Our frustration at lack of evidence in such matters should not lead us to be too free and easy with allowing speculation, however well-intentioned, to take its place. Creative use of sources and dogged research can turn up insights, but historical method reaches points beyond which it just cannot go. We may find that crucial letter from Ginevra Malatesta commissioning a pack, but we are unlikely to find any evidence ever that an anonymous artist's mother was a key influence, even if it is was the case.
2.
Being derived from the hieroglyphs of ancient Egypt, these Letters [the Hebrew alephbet's] are themselves the so-called Book of Thoth whose title is, by extension, conferred upon the Tarot.
This seems very questionable. The Hebrew alephbet derives from the Phoenician and Proto-Canaanite, not some simplification of Egyptian hieroglyphs or demotic. Ugaritic is an entirely different kind of script (cuneiform) but has the same letters with a few extra--some 28 or 30--which points to the fact that this alephbet precedes the script and is not Egyptian in origin. Again just because we are dealing with historical time does not make such a conjecture historical.
I am generally a reader rather than a contributor, but I would support the creation of a separate forum as the most appropriate way to offer a proper arena for more speculative ideas. They may the interest of a minority, but to this minority, including me, they are of great interest and I am sure that most readers of this forum would also be readers of that one, just a little more relaxed about standards of evidence.