I agree with everything you said KGB. But given its open source what stops someone from stripping out the mandatory fee and gaining the benefits without paying.
The share chain solution is that if the majority of p2pool are willing to play by these rules and reject freeloading shares that don't include the development fee then it would be enforced. That would be a hard fork so would need managing.
Will the majority of miners play ball though?
I don't think there's anything stopping someone from re-coding it to remove the donation, however I'm not sure how the permissions are setup on the main git repo, whether those changes could be re-submitted back into the main branch without forrestv's approval. That might require a fork.
Anyone is free to fork the code and make their own changes, but I'm sure also if someone did something like that with nefarious purposes it would be found out pretty quickly and that node could probably be blocked from interacting with the rest of the nodes. Now I say that without knowing for sure, but that would be my guess.