I would imagine all of the Halo assets default to Microsoft instantly and it would be entirely up to them to sell them again if they see fit. Although I think some of the Halo models were contracted out because they were plastic and Spartan doesn't have the machinery to do in-house plastics.
I haven't seen the paperwork or anything, but usually, the moulds and models would be the property of Spartan Games, but you'd need the license to sell them. So anybody wanting to resurrect Halo would need to buy the designs/moulds/masters/stock from Spartan, and then sign a licensing deal with Microsoft, and wouldn't be able to do anything with the Spartan properties unless they had it. MS could always buy that stuff from Spartan and do it themselves but they obviously never would.
The question of the value of the DW and FSA IP is pretty open. It clearly has a solid, if very niche, market and it doesn't have much for direct competition. But the thing about gaming systems is that most developers have way more ideas than they have the ability to make them (see Spartan...). So I would wonder what gaming company would buy someone else's dead baby when they have many ideas of their own that they haven't got out yet. A lot of the initial work is done, but it would be hard to put your own spin on the system without destroying what value the IP has, the existing players. Change it too much and it isn't the same game any more, but leaving it the same would make it harder to embrace it as a developer.
There are a lot of companies who wouldn't mind adding those lines to their portfolio, but you'd need someone with enough heft to add a pretty extensive line, who also had lots of capacity and experience in resin casting, and has some money on hand. I'm not sure who fits that bill.
I think that rewriting the rules to go with the existing models is certainly possible. Warzone changed the rules and also the models, and I guess they're doing okay with it. As long as you have a well supported product, I would expect people who already have fleets on hand would probably mostly just roll with it.
I expect that an Epic style community rules revamp would be mostly the current ruleset with some tweaks and fixes, because that's the easiest thing to get everyone to agree on. I would want to start from scratch, myself, as the DW rules have a bunch of issues which are fundamental to the rule concepts (but that's just me).