2. You are taking two seniors to do/see kids' stuff? universal studio ==> disney. Once you take out all the rides, not a whole lot left for two seniors.
3. most likely 会太热. Check National Park website.
If I were you, I would also look into the air ticket price outbound LV and return from San Diego or LA (open jaw ticket). Ticket price may not be that much more expensive. Of course, be sure to check with your rental car company about their drop off charge at a different location (there may not be any and sometimes rental car dropped off at a different location may even be cheaper on the rental charge). With the high gas price these days, it does not make sense to drive back to LV just to catch your flight unless the rental car drop fee is outrageously higher.
Just my 2 cents.