Give him a break, this is how projects - and especially personal projects - develop. You find a cool idea you want to explore on, the end product might be something totally different then what you started with.
Yes, but you don't typically replace the same domain with two totally different services. You create 2 new domains. I would never pay someone for a service when they've repeatedly shown that they're willing to throw out the system people have started depending on and replace it with something radically different. Not only can I not trust the stuff to stay around, I can't trust the developer to have enough of an attention span to care about the old stuff enough to even just leave it alone to keep working in maintenance mode.
The hook.io domain actually expired and went to public auction recently and I was able to purchase it.