Are you worried that heating the cured epoxy might release hazardous vapors? I don't really know for sure, but I've done it many times and I'm still alive and have all my reasoning faculties as far as I can tell! I imagine the small amounts of material we use and the fact that we only do this stuff occasionally rather than regularly means we're not going to absorb toxic levels of chemicals, as long as you use common sense. Of course it's best if you're baking polymer clay in a dedicated oven and not the one your meals get cooked in.
Epoxy does soften at high temperatures, and then hardens again when cool, so there is a possibility that if you put very thin Sculpey over epoxy putty it might distort. But as the Sculpey is going in the other direction, i.e. hardening, and as this process is only for 10 minutes or so, it should be fine.
When I need to soften epoxy, as I was doing yesterday to release an aluminium tube, it took about 15 minutes of heating up the tube with a hot air gun to soften the epoxy.
I don't think hardened epoxy releases vapours significantly. The process is that the molecules of the resin and hardener link together to create the final polymer. Once they have done that and hardened they are effectively no longer toxic. The only problem occurs when there is more of one than the other. Epoxy also has an interesting characteristic, in that it continues to harden throughout its life. It is possible to post-cure epoxy by raising the temperature to around 45 deg C for about 8 hours or so, and this can increase its strength. So increasing the temperature to 130 deg C for ten minutes should actually make the epoxy putty a bit stronger.
I think that's proof of the pudding, then!