acetone and styrofoam

Acetone is not the best solvent to work with for polystyrene as it will evaporate very quickly. Something like toluene will be much slower to evaporate and give you more working time. More solvent will make the whole "melted mess" thinner.

As for refoaming it, this can be challenging without the proper (expensive) equipment. There are thermally inititated foaming agents that you could try and mix into the solvated polystyrene, heating the polymer once the solvent is all evaporated. These are generally based on nitrogen generation from a (di) azo compound.

John Aspen Research, -

Opinions expressed herein are my own and may not represent those of my employer.

Also you can expand polystyrene with isopentane, which is how foamboard is made. You need some pressure while it is mixing, and then it exands by itself as soon as it is exposed to air at ambient temperatures.