The FX Lab's Tutorials section has some stuff on silicone seaming & painting in the stickies;
Google and YouTube ('paint silicone mask' etc) turn up plenty of others, either directly or linked from results.
Annoyingly I was looking up loads of this stuff just last week, and now I can't find half the links / videos again... Loads of stuff on latex & gelatine, but not silicone.
AFAIK silicone and latex won't stick to each other, so using silicone to fill in the seams wouldn't work (and you couldn't paint over it with latex paints, either). Careful use of nail scissors - the small ones with slightly curved blades - should let you clean up the seams with very little need for filling.
My advice would be NOT to fill the mask with foam. That would be a non-reversable decision. Make a headform, or use a commercially avaiible one altered to fit the inside of the mask. That way you still have a nice mask to use if desired, and a wonderful display bust. I like the stand design, but I bet you can do better and not rip-of the commercial design.
I did I thread on something like this a couple years ago: http://www.therpf.com/f9/finishing-raw-michael-myers-mask-tutorial-76124/#post1067435
Give me a yell if you need help. I can give you some advice either through PMs or the phone. Ship me the mask, some liquid latex, and a couple bucks, and i'll even do it for you.