Actually it was the second changeover at 3-2 (Canas got the break at 1-1). It didn't have any impact on him, and I doubt Federer thought it would have much of an impact, he knows Canas' mental strength and also knows about his results this year, it's not like he's only just starting to play tour matches again considering he already has a tournament win.
Pretty sure it was at the first one, at 1-2*. Check the live scoring thread.
As for the rest, well many players have felt the pressure from being at a winning position against Federer, and it took much less than injury timeouts for that to happen.
Canas feeling the pressure, that would be perfectly possible. He had not played against such a high-profile player since his comeback, not even against a top 20 player if I'm not mistaken.
I agree about Canas' mental strenght even though he has already lost a match from two sets and 5-1 up in the third in Davis Cup, many years ago.