Mayo needed 15. Taking 7 then 8 does not change the fact he needs both. He absolutely made the right decision. Analysts get this wrong on TV every week as though they're new to this 2-point possibility.
If the Pats are unsuccessful on their 2-point try it doesn't matter whether it's earlier or later. If they're successful and they scored the other necessary TD, they could go for a win.
It is completely irrelevant to "stay within one score" when either way you need the 2 point conversion. Missing it means you need three scores instead of 2. Waiting to find that out doesn't change that.
Now Pederson on the other hand failed to make it an 18 point game which would have ended the game. Getting 7 there instead would not have benefited him. He's clueless.