I believe that your rifle was made before CVA started building them like this:



At least that increases your chances of removing it.
Other folks have swapped out their drums, but that doesn't mean that it's easy. Even frozen revolver nipples can take days to get unstuck. One method they use is tocook it ina container of transmission fluidover a hotplate, let it cool, and cook again while soaking itover a period ofdays, or until they are successful however long it takes.
Soaking in kerosene or another distillate might help too.
Then try a goodfitting wrench, or drill ahole straight throughthe nipple holeandstick asteel rod through it,maybe evenusea cheater bar toadd some moreleverage.
While boiling water was a good try, it won'tpenetratelike a hot oil will.
Some folks describe a heat & then freeze method, orrapping it with something hoping that the shocks will help break the bond. Even using adead blowhammer to increase the strength of the blowsand not damaging the stem and causing it to twist offisthechallenge.
Maybe that's why sending it in to CVA is the best solution if you can't get it off yourself.
I always remove the clean out screw and try to clean the drum out the best that I can so it doesn't accumulate too much gunk over time. Idon't see how it would hurt to try to keepit clean, but I can see how it could hurt by not regularlyremoving the screw and cleaningout the fouling through the most direct route that it provides.
Drums aren't quite the same as the TC patent breechwhere theydiscontinued installing the tinyclean out screw. They probably just didn't want to dealwith thestuck and lost screws under warranty anymore anyway.I don't know of any drums madethat don't have a cleanout screw, and it's to their benefit that they do have one. If the lastowner kept it clean maybe this problem wouldn't have ever occcurred.