If option #2 is your definition, then if you have a complete separate set of planes (an alternate prime, connect to alternate inner, etc.) it could be justifiably called an alternate multiverse I think.
Admittedly, if the people involved with D&D end up defining their use of 'multiverse' as you state in Option 2... then sure, it is what it is. But if they do that, then they are schmucks-- they would be taking a word that means something specific and changing it for absolutely no reason, and which only clouds the entire idea in the first place.
A 'D&D Universe' (not Multi-- Uni!) would include a particular person's Prime plane, alternate and variant Prime planes, Feywild/Shadowfell, elemental planes, Elemental chaos, ethereal, astral, and outer planes. That is one specific Universe because it includes EVERYTHING that that player has access to. Or indeed if the person isn't using the Great Wheel, then it could be something like their Eberron/Siberys/Khyber and the 13 aerie planes make up their 'D&D Universe' if they play an Eberron game. And if you take MY Great Wheel game Universe and YOUR Great Wheel game Universe, and Keith Baker's Eberron game Universe, and Matt Mercer's Exandria game Universe... all of them together make up the D&D Multiverse. They are all alternative worlds and Universes within the umbrella of D&D.
Now yes... to go along with and agree with what you have mentioned in your Option #2... I myself have heard Jeremy occasionally talk that while all of our individual games make up variant Prime planes to each other (and characters can move back and forth between them if they so wished-- which is the explanation of the 'Greenwood/Elminster/Mordenkainen convos in Greenwood's kitchen' stories which have been explained away)... he says there is but
one set of Outer Planes and that any characters (like Tiamat) that appear in different settings/worlds are merely mirrors/shadows of the ONE actual Tiamat. Which to me... is absolutely stupid. No, there is not only a SINGLE Tiamat in all of D&D, because that is not something Crawford/Perkins et. al. get to control. They don't get to decide on the Multiversal "reality" of any of this stuff, just like none of us can. Which is exactly why the Multiverse concept exists!
As an example of that stupidity... if Crawford and Perkins were to decide at some point to legitimately kill/destroy/remove/purge the "One" character of Tiamat from Dungeons & Dragons-- all that accomplishes is remove that character from anything further that THEY produce. THEY can choose not use Tiamat going forward, but they can't stop anyone else. They can't stop someone from running the
Tyranny of Dragons adventure after the fact. So if someone plays that adventure, what is that Tiamat? Does that Tiamat character not actually exist? Because THEY chose to remove Tiamat from the game? If Jeremy et. al. suggest that... that's pretty much saying that all of our games don't actually exist then. None of our games are "real" because only they get to decide what is or isn't real. Which of course is ridiculous, because the entire point of the Multiverse concept as they've been using it up to this point has been to explain why all of our individual games ARE real. As real as anyone else's! My game is no less real than Jeremy's game.
And to pile on top of that silly assertion of Jeremy's (assuming he does in fact believe that idea of "Only one true Tiamat, but with lots of shadows" still)... what happens in 15 years after Jeremy and Chris are no longer on the D&D team and the next production staff decide to bring Tiamat "back"? Well, doesn't that then prove the "only ONE true Tiamat" idea was bull? Chris and Jeremy could no more remove the character from D&D as any of us could. If one of us "kill Tiamat" in our games, that means nothing to the great populace of Dungeons & Dragons... just like it means nothing even if Jeremy, Chris and the D&D production team say it.
The ONLY way that any of this works... the only way we can legitimize every single player's game and table as TRUE and REAL... is by using the concept of the multiverse as it is meant to be. Every single one of us (including Jeremy and Chris; including Rob Heinsoo, Andy Collins and James Wyatt; including Monte Cook, Jonathan Tweet and Skip Williams; including Zeb Cook; including Gygax and Arneson), we ALL have real and true D&D games in our own individual D&D universes-- where whatever happens there happens-- but which doesn't actually impact or affect anyone else. We are all alternative universes to each other-- everything different, every different setting, every different timeline within these worlds and settings, every single cosmology-- we ALL have our own thing in D&D that is no more or less real and true than anyone else's. And when taken as a whole... we ALL make up the individual universes within the entirety of the D&D Multiverse.