Thinking about it, you're probably right that this is what they intended. I don't really like it, though. From River's point of view, her own past has always been like this. From her point of view, she always grew up as Mel. As far as we know nothing in her own timeline has been altered from how it always occurred. The very fact that River Song exists in the future says that she always grew up that way, which should mean that she's always been there. My internal sense of time travel says that anything that a time traveler does should already have been incorporated into the present and the future. But I am willing to concede the timey-wimeyness and give it a pass, because it does seem to be intended this way given the Doctor's comments about people at the wedding. And Doctor Who doesn't always work the way my internal sense says it ought to (the most recent Christmas episode is an example). Specifically, the Doctor's actions through his own personal timestream don't seem to take effect until he actually does them in most cases. I'm not sure if that rule is consistently applied however. It certainly wasn't in season five, when his future self came back and changed past events while his present self was still trapped. But perhaps we can attribute that to the end of the universe. Anyway.

Possibly, but would the death of a double be referred to specifically as a fixed point in time? That's the same terminology the Doctor uses to describe Big Important Events that can't be altered. And again we have the question of whether or not the flesh Doctor is capable of regenerating. The Doctor who was killed in the first episode was beginning to regenerate.

I'm interested in the fact that the Doctor knows when he's going to die and who's going to kill him (or thinks he knows, or appears to know). If it was indeed the real Doctor in the first episode, did that Doctor also know? My internal time travel notion says that if he knows now, that means his future self knew when he went there. This means he made no effort to avoid it. He did, however, bring Amy, Rory, and an older version of River there to witness what happened, and apparently invited his younger self, which suggests that future Doctor believed events could be altered somehow. Yet at the same time he didn't want to avoid those events for some reason. Summary: no frigging clue.