To me . 98% yes.

Literally all the evidence points to yes imo.

Probably. Even apart from all the lore evidence (which honestly isn't certain - theories can be wrong and our guesses can be faulty), there's the fact that Hoyo seems to be aggressively promoting him. He features too heavily in all kinds of official stuff (Mavuika's web event, 5.3 promo art, tribute video + song, Paimon's Paintings, trailers...) for him to not be playable. And that's not even getting into how they've been hyping him up through in-game dialogue/voicelines/etc since the Fatui intro video was released too.

Gacha games make money by selling characters. This means that characters are their product.

You promote a product because you want people to buy it.

You don't promote something you don't intend to sell.

Hoyo wants us to like him. They want us to pull for him. Based on the polls I've seen, people want to pull for him, both the EN and the CN fandom. His popularity is frankly crazy, matching or even surpassing both other Harbingers and Archons, which all tend to be insanely popular anyway.

It'd be stupid to pass up on that kind of profit.

Although.. I guess you COULD say their doing it just to keep the fanbase sated. Give them hope to keep playing but never intend to actually release him.

Might be just be, but that seems highly likely to backfire.

Yeah...

But. If that DOES happen, eventually they'll get massively review bombed and popularity will definitely plummet/people will stop spending money. Which will Or would be enough incentive to change their minds. (although that's awful)

Very much possible. He has a great design, enormous and simpy fanbase, and Hoyo wouldn't let all that possible money go to waste. Besides, it was heavily highlighted in the quest that he didn't explicitly die, but rather was merged with the Lord of the Night, so I think he will be playable.