Yes, the easiest way to acquire magic power is to contract with Regret. You will receive immortality and a handful of synergizing abilities in the bargain.

As for whether you can have more than one patron, absolutely. That's the premise of this RP, in fact - and it's to demonstrate the reason why people usually don't contract with more than one patron. That reason is because, if your patrons' priorities conflict, the consequences could be deadly for the contractor. Now, if you know from the outset that you'll be taking on multiple patrons, you can certainly include language in your contracts to resolve any conflicts that arise. So there are safe ways to do it if you come prepared.

And yes, any preexisting contracts would still be valid unless stipulated otherwise. It is the contractor's responsibility to ensure all their patrons are satisfied.