To your first question: I had the exact situation with one of my former clients. The product didn't take of but still, i support them and if changes have to be made on their product i am still the one who does it. I guess, i would treat this new customer the same. This is basicly one of the reasons that i cannot assure more than 3 days - i sometimes want / need to work for my previous customers.

Second: my feeling is that there was nothing bad going on and the reason that he left is that he had to travel 2h twice every day. But i certainly will check that.

Third: I don't like to compare salaries but i think it is beyond avarage in this town. The problem is, that Hamburg is not quite the tech city, it is really hard to find good devs here.