It's been established, multiple times now, that the current Cap is the real Cap.
Pretty much this.
Um, not to point out the obvious but almost every single character who exists within what we know of as the 616 today is 'reality warped', based off a different 'reality warp', based off the original. Or did we forget that Franklin Richards has had to restore the 616 not once, but twice? The difference between Franklin's powers and Kobik's is negligible.
Forgot to add that, currently, Tony Stark (with an AI brain) and the X-Men (all of them, the entire group, as a whole) are, themselves, clones. So singling Natasha out does come off as a tad hypocritical.