For night 2, I assume you already know the North Rim facilities will still be closed then. I'm guessing this includes the campground store. No big deal, unless you were planning to buy something.
daw37 wrote:Its days 1,4,5 I'm a little worried about
I won't comment on days 4 and 5 because I'm only familiar (very familiar) with the Main Corridor trails. But I wouldn't worry too much about day 1. South Rim to Cottonwood isn't too extreme. Days 1, 2, and 3 are not unreasonable, provided you're up for those distances with what I assume will be a heavy overnight pack.
On day 4, you always have the option of altering plans, like you wrote, and going up BA, skipping the Hermit trail, camp, and extra day. You can make that decision based on how you feel that morning at BA Camp. Or you could even wait until you get to Indian Gardens (3800 ft) to decide, since that's the last point common to both options.
In the first week of May, the heat shouldn't be too much of a problem, unless you're sensitive to warm temps (profile says U.K., haha). It generally doesn't get hot until the second half of May. Average highs in the lower elevations for your dates are in the 80s F, and it won't be humid in May except maybe after recent rain.
Note the nighttime temps on the North Rim. Average lows are in the lower 30s F. No worry, as long as you have the right clothing.