How much a PT makes involves alot of variables...For example - How many clients you get, how much you charge, gym rent...It all depends on you really as a trainer.
It depends where you get the cert. The one I'm doing at the moment is actually a lot harder than i thought it would be. I'm doing mine at a college. If you do it at an institute its generally easier and the course may be shorter. But again, this is only where i live, it might be different in your location.
I suppose...you could...do that. But seriously, if you're a GP, would you want to put in the extra hours being a trainer as well? Well, i know that i wouldn't and i'd imagine a GP makes a fair amount of money without worrying about this.
If that scenario that you posted came up than the doctor would likely recommend a PT for that patient to see, however if the doctor is going to recommend them to you, you obviously need to have a good relationship with him/her and you do that by referring clients who are a health risk to that doctor and calling the doctor to see what they have to say etc. See how it works?