Best time of day to get hitched with an almost 2yo?

My advice is to not try and arrange your timings. Trying to second guess their nap needs is going to be very difficult and especially like you say the excitement of the day which they may or may not pick up on but will certainly be effected by their parents change of routeen and activity so they may not get their normal nap in beforehand with the parents getting ready / travelling. Or they could be the type of child that is not disturbed by anything and when they sleep - they sleep. kids are so different anything can happen, could even be ill.

Many times I have seen a parent frantically pushing a pushchair up and down the car park saying they (baby ) are usually asleep my now. Or during the photos I hear "they didn't get their usual nap earlier"

You could put it past their parents but by the time of your wedding your nephew will almost be twice as old as they are now.