Help with finale of season 6. (Spoilers)

So last night i just finished the finale of season 6 and there are a few things bugging me.

First, the replacing of the doctor with the tesselector. While this is a useful method of saving the doctor it does not negate the fact that the doctors death on 04/24/11 is a fixed point in time. When Riversong changed this the first time it resulted in the destruction of time itself. So why is it that this is okay? Or is it that the fixed point in time is the recording of his death on that date or the action of trying?

Also if the doctor that got shot was the tesselector, was it the same one as shown the first time the doctor got killed? Or was that the real doctor the first time? If it wasn't then how come it showed the beginning of the regeneration process?