It makes no difference, watts are watts regardless of the working voltage. A heating appliance will just cycle off more often if you're using a higher powered one (big oven -vs- toaster oven). However, a large oven may use slightly more power than a smaller one because you have more mass to heat up in the large oven. But a large oven may be better insulated than a toaster oven, so you don't really know which is better.
The only reason to not use more than one appliance at a time would be because your electrical service is too small and you blow fuses. Cost is the same -- you can use 2 KW for one hour, or 1 KW for 2 hours -- the total is 2 KW-Hr in each case.
Turning up the AC won't help save when using an oven simultaneously unless you don't plan to turn the AC back down again. Your oven is dumping more heat into the house that will either dissipate (when it gets cooler at night), make you hotter than you want to be, or the AC can get rid of it (but that costs money). If you're trying to cool the house, I consider cooking outside on a BBQ or perhaps just cooking on the hot asphalt.
I wouldn't worry about any of the things you've mentioned, as the cost impacts would be minimal on your bill (except perhaps using the oven at 450 degrees all day long with the air conditioning on).