Welcome to the forum!
If the stocks that cracked are a Thompson/Center product, they have a lifetime warranty and they'll replace them for you.
As far as muzzleloader breech plugs are concerned, my advice would be:
1. Go stainless steel in the first place.
2. Make sure you clean and lubricate it well after shooting.
3. If the breech plug really gets stuck, thoroughly treat it with something like Prolong, Prolix, Kano Kroil or some other product before you try to remove it.