Trek fans take more time, effort, and thought in apologizing and rationalizing these flaws (it's a sort of game we all play) than the writers do in laying out their stories. Honestly, I am more impressed with your defense of Orci's script, than anything else. I really like the river metaphor. It's simple, plausible, and although it is also equivocal and masks the very problem at issue, if someone had something like this in the film, I probably would've kept munching my popcorn without immediately noticing this detail!
Given how impressed I am
by most of your post and a previous one I commented on, the above seems kind of like it was written by someone else. Sure, Greg's
river metaphor may be poetic, but not plusible I believe, as it seems to me it overlooks the second law of thermodynamics
(a point you yourself more or less made I thought). Nor do I think it would just mask
a problem, I think it would be
a problem. One sufficient to curtail popcorn munching in it's own right!
The supernova business already gave a false impression of that phenomena of course, but these are not faults that can be excused because it's Sci-fi. Not without some explanation in-movie as to why "normal science" doesn't apply.
Or... Better yet... don't write yourself into narrative corners! ...
That would be my preference.
By the way, I found the comment about the "timestream's way of mending itself". It was a line said by Old Spock to Kirk which they cut from the film (mentioned in the commentary starting at 1:17:35 or so).