With all "proof" listed I'm still not convinced it's BG3.
However…
why have a Belgian company do the sequel to a Canadian developed D&D game? Is D&D even big over there?
I refuse to play mmos so dunno. Maybe Corwin could answer how many belgians are in his DDO guild.
Because the couple of years old, forgot deliberately it's title, d&d mmo got shut down, it's now impossible to check how popular it was in Belgium. Maybe it was popular only there, but Belgium doesn't like modern mmo "monetizations" that are scammy so there.
That aside, the answer on why would anyone do a BG sequel is simple. It'll sell.
Assuming it's a real sequel and not yet another mmo.
Did wotc have change of heart and bless something that isn't mmo noone can know except them. That's the reason #1 I'm not convinced.
I'm not sure why you think Obsidian wouldn't have been interested, but like I said, I don't think they would have been able to outbid Larian anyways.
Obsidian's interest + Unity… Won't impress anyone.
Larian has it's own, similar but a bit better isometric engine.
Obsidian can outbid anyone thanks to it's new owner. But why would they? In some future Microsoft will buy both Bethesda (for singleplayer FO continuation) and Bioware (ME and possibly BG continuation). It's $ from singleplayer games we're talking about here. And Microsoft needs strong singleplayer exclusives for it's plastic trash named Xbox, as you've probably heard Sony's plastic trash pwned them with singleplayer exclusives (HZD, GoW, Spiderman).
If you were a newly appointed decisionmaker in Microsoft and checked the past, remember ME on xbox? Add a timed FO5 exclusivity and you got your upcoming console generation covered.
Unless of course the rumors are true where Microsoft and Sony plan to "end the war" and cooperate due to Google's upcoming games streaming service. That public bus/train service won't kill comfy private cars (PCs), but the mushrooms junk will be trashed.