Insulating the concrete attic with rockwool slabs
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.