On the sabots, I believe that the one you need is green; using a .451 sabot with a .429 or .430 bullet can let the bullet slide right down the barrel while the sabot stays put ,this is then a barrel obstruction and could cause damage to the gun at the least and possable injury or death at most. Please be careful of this situation.
As for the build up in the flash hole portion of the breach plug there are several guns that have this problem; I have it with a TC Triumph if I use the Win 777 209 or the Ren muzzleloader 209, I can use the win regulas 209 for loading shot shells with out the problem.
And the horrable crud ring, it does not get real cold where I am living now so I can not say much about that , however I did find out by polishing the barrel that the higher polish will not crud ring near as much as an unpolished barrel it [follows that as a gun becomes polished from break in and use it should improve this is my assumption any how I have polished the only one that got it right away so I am only guessing about this] I know some get it and some dont in very similar guns and how polished they are from the factory and how polished they become from a deliberate polishing or use will have an effect on it for sure.
If you continue to have the breach plug problem keeping an old fashioned hand crank drill for small drills makes life much easier.
Hope to have helped or at least given food for thought. Lee