If GHSP is oil based the silicone won’t cure.
You shouldn’t be heating silicone. It dries quickly in a warm room. I just started painting it, I might hit it with a blow dryer to speed it up but it dries pretty fast, gives you time in between to plan the next layer and get ready.
I’m struggling to paint veins…
Any suggestions?
Thin the paint down a bit for the veins. I have found the best thing to use is the NOVOCS. I get the gloss version because you can clean the silicone with it as well as thin the paints with it.
If you do not have NOVOCS you can use a little odorless thinner but using too much odorless thinner in your silicone mix can cause it to peel down the road so keep it to a very minimum.
Small world!
Where do you find the bits and pieces for practice? I’ve looked but haven’t had any success.
I stalk eBay, this forum, etsy, and doll fan. With any shopping experience, make sure you’re getting the real deal. Ask the sellers questions and filter out international items so you dont get stuck with a China fake. Fakes arent as big of a problem with this forum and doll fan, but it’s good practice to ask when you’re not sure.
Irresistables has some practice faces that are pretty inexpensive.
On Ebay I set up a search alert for “silicone booboo” and have found great deals that way.
Just be on the lookout and embrace your inner stalker. 
macphersoncrafts.com has silicone practice faces and starter sets of silicone paints. irresistables.com also has them.