I am not nearly as convinced as you are. First of all, how monsters and how PCs work is unrelated. There's NOTHING in the DMG which says one way or the other, show me text that says a secondary weapon grants some certain number of attacks. Again, Deities and Demigods isn't speaking about PCs at all, none of the stat blocks there follow PC rules in any consistent way. It isn't in UA either, any more than it is in the DMG, in fact UA has nothing to say about TWF. I'm not finding anything in OA that is specific to TWF except the Kensai 7th level ability, which still doesn't explain number of attacks. I have never read Lankhmar.
As for the article by Roger Moore, not having gone and dug it out, I take your word for it that Roger interprets the rules that way. It isn't even that it is an illogical interpretation. It is just NOT THE ONLY ONE consistent with what is written. It isn't the only one that was prevalent during that time period. I guess we could consider Roger to have established a canonical rule? In general Dragon articles are merely optional rules, essentially homebrew.
So, I'm only disagreeing that we're able to say exactly what will happen if you wield 2 weapons. Even if you assume it grants another attack, you have no way based on any of these sources AFAIK to say if it is per round, per 'attack per round' or how it interacts with thrown weapons with higher rates of fire (with our without UA). All we know is that in 2e it grants an extra 1/2 of an attack/round in effect (even 2e is ambiguous about how this works for rogues from what I recall).
Look- I would point out that you sometimes tend to read back the interpretation from 2e back into 1e. Like with contingency (which you have mentioned several times now in reference to 1e, but ... was never introduced until UA in 1985).
But to make this glaringly obvious-
You have material from 1978 written by Gygax showing how TWF ("Attacks With Two Weapons") works. Because he wrote the drow section, which had the whole thing where it showed what happens when you have a weapon in each hand. You get an attack with each weapon. It wasn't in any way unclear, and it was the predicate for a whole series of modules!
Then, in 1979, you had the actual rule in the DMG. It's not unclear. Why? Here's the beginning of the rule:
"Employment of a second weapon is always at a penalty. The use of a second weapon causes the character to attack with his or her primary weapon at -2 and the secondary weapon at -4."
Get it? You attack with your primary weapon in that round at X, and with your secondary weapon at Y. Later it refers to the "attacks" in the round, and that you cannot use the secondary weapon as a parrying or shield (which, again, can only make sense if it is being used to attack
that round).
The next year, we have the publication of Deities and Demigods, which expanded the range of permissible dexterity scores beyond 18, and showed that if you had a sufficiently high score (19+) you could attack with two weapons with no penalty- with examples in the book. The examples, of course, got the attacks in each round.
Then came the Fiend Folio in 1981, reiterating the rule for Drow-
the same rule that was reiterated in 1985 for Drow as a PC.
If this was unclear, you had Dragon Magazine, the house organ of TSR, publishing an article by Roger Moore (you may be familiar with him ....) in 1982. This was not a obscure article- it was reprinted as a "Best of Dragon Magazine" article as well. Anyway, the point of the article was two-fold; to expand the number of useable weapons (originally just a hand axe and dagger) and to deal with a few edge cases. Such as the 3/2 (Fighter get 3 attacks in 2 rounds with one weapon; what happens with two weapons, and when?). Notably, it treated as the baseline rule from the DMG that "Characters using a weapon in each hand will effectively double the number of attacks that they may make each round ..."
By 1985, and OA (Kensai, et al), UA (Drow, et al.) and Lankhmar (Grey Mouser, et al.), there could be no reasonable dispute about the application of the rule in 1e. Of course, there could be no reasonable dispute about the application in
1979, and these rules were just simple extensions of the baseline (giving it as a special class ability, giving it as a special racial ability, and giving it as an ability within a campaign setting).
I am going to put this again nicely- there is, quite literally, NO OTHER INTERPRETATION of the rule in the DMG. If you have, say, an illusionist with a high dex (but I repeat myself) who is dual-wielding daggers pursuant to the DMG in 1e, what other possible interpretation could you have of the rule other than the illusionist attacks 2/1?
So when you say that there are other reasonable interpretations, I have yet to hear one. When you add in the sheer number of other sources, I am completely befuddled by what you are saying.