Thanks for your reply.
For (1) you could do merged-mining, as it costs practically nothing extra to miners. But then you have to be careful to keep interests aligned - the new chain can't be competing with Bitcoin, and should in fact be seen as complementing it in some not insignificant way.
Okay, I think I understand, but with merged-mining the new chain would still have a currency of its own that functions as a reward for miners, no?
(2) could be addressed using identities that have a cost to create, and can be blacklisted if they are abusive.
Yes, good point. Who would blacklist them though in a decentralized network?