So, I've been playing as various Italian Dukes, and I run into the same problem. I get a good early start, get up to ~8 Demesnes, and some nice vassals, and then the HRE gets absolute authority, and I can't do anything.
I can't wage wars against anyone, even with claims.
Rebellions against the HRE never work, b/c the emperor can move his massive stack back and forth across Europe with no attrition. So, the game just stagnates.
Even if I try to fight the emperor myself along with multiple revolters, the HRE can still raise levies from my vassals and demesnes, and you can't beat those massive stacks.
Anyone else see this? Any way around it?