I'd add that if draining some of the fluid out might as well go ahead and do a partial fluid change. Drain out what's in the pan and refill with fresh fluid of the correct type, being careful not to overfill.

Be sure to check the level as specified in the owner's manual. Usually that's with the car on level ground, engine idling and transmission in park or neutral. There may be hot and cold marks on the dipstick, if not the level should be checked while hot.

The only caveat here is if the fluid is actually gritty it is best not to do the partial change, just drain out some and fill back to proper level using the old fluid. (Grit would mean clutch and band material is severely worn and floating around in the fluid. Under those conditions fresh fluid could make the slipping worse. I know Scotty says Toyotas don't usually do that but I'd still check.)

If you're absolutely, unequivocally sure that the tranny is overfilled, there are two ways to do a partial drain.

1. Slightly open the drain plug while catching the fluid in a clean pan. Then check your fluid. If too much was removed, then pour a bit back in via the dipstick.

2. There are pumps with hoses that can pump out fluid via the dipstick. 

As for the missing bolt, I can't comment, since it's impossible, for me, to know the location of the bolt. Many bolts on that generation of Camry are 10mm. If there's a companion, remove it, take it to a parts store, and match it up.

Good luck.