I have little to contribute to your questions on this one of a kind car, but I do have a little information.
The floor in Coach #280 and #284 were and are painted tuscan wood plank flooring. I believe the linolium tiles were not used on standard platform coaches, but I have no idea what car #313 had for flooring as the "Silver Vista".
Not to argue Kev's point about the subfloor under the sills, I don't think there was any insulation material other than dead air between the sills, which was itself an effective barrier from the cold.