Yes I have used a fine razor saw which gives a nice clean surface but you\ said:Polystyrene adhesives are solvents that work by dissolving the plastic, so that when it evaporates, the two parts have effectively become one piece. So "ungluing" probably won't work.
I sugggest a fine razor saw or very sharp blade for separating parts. A small screwdriver ground down into a chisel may be useful.
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