Well, the corks shouldn't be steamed or otherwise heated. I bet that is why they fell apart. Just leave them be, I guess, or remove the corks and recork with untreated corks. I use #9 corks, which are bigger, so your #8 corks are pretty small.
Dont' use the PVC shrink wrap, until you're ready to give the bottles away. You want to see the corks, to make sure they aren't poking out or failing.