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?
No. Remember: it costs practically nothing extra to merged mine the new chain, so the incentive to mine doesn't have to be very large - complementing (and not competing with) Bitcoin in some not insignificant way could suffice.
(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?
If one of your peers is being abusive,
you would blacklist it. Notice though that a single identity gives you the ability to abuse every node once - ideally you want it so that abusing one node gets you on every node's blacklist. Distributing cryptographic proofs of bad behaviour might a way to achieve this.