Best practice for filling a pit in a concrete slab

Just throw any insulation material you can get on your hands on the attic floor and cover it with "vapor-permeable film" (I don't know the correct term here) if it's not waterproof like styrofoam. Make sure the film is laid proper way up and won't move. Of course, this assuming that you won't be using this attic at all (not just "almost") - such insulation won't survive being stepped on it.

Because you're going to tear this house apart soon, the quality doesn't matter much. I also doubt any insulation would be reusable after 5 years, I think you should treat it as disposable.

I think it's going to be effective, as ceiling is where the most heat is lost. I've seen many houses which were insulated like this - just styrofoam slabs tiled on unused attic floor. I haven't heard about the flea problem - any insulation is certainly a nice place to nest, but fleas also have to eat somewhere.

Level it... with more tile?

I've done my share of concrete work, but there's no way I'd trust myself to float a finished floor and I wouldn't personally guarantee any work I did less than four inches thick. I'd tile it just like the rest, if I were set (haha) on doing concrete, I'd hire that out to a stone flooring specialist.

I doubt you own a commercial floor buffer and a 20amp grinder. Having only seen it done, I can't go into great detail. I just know that I don't want to grind on a floor for two days straight. IMO, tile is your best and cheapest answer: anyone can tile well enough and your labor is free.

Note: "Self-Leveling" is a name, not an adjective. Call it whatever you want: it doesn't. You still need to know how to float concrete and you'll still need a floor buffer to correct any mistakes.

Patching a slab so that you can tile over it is DiY. Installing a Stonhard floor is best left up to a professional company (of the same name, with which I've no affiliation but I did know some people that worked for them, who said the job sucked, but the dust completely coating the interior of their car told me that).