1. Depends, but likely a lot if you're capped. It increases with each conversion you do. I'd expect in the millions for sure.

2. You can't remove the silvers once you put them in, no. They're gone.

3. It would only take one cleric of Voln to create the intercession gem for you, or 3 liabo aligned clerics to cast at the gem if there's no Voln cleric who could create it.