We were there last August and we will be there again this August (both times thanks to
free dining). Some tips for the heat that we used are:
1) You got it already, Early EMH, especially at MK. I went to AK once for early EMH and it was insane, but I think maybe because that was the Saturday before Easter and there are few rides open than at MK.
2) The stores towards the front of MK and MGM are connected. So you can walk straight thru in the a/c and without crowds. The "buy me this buy me that" is the a non issue b/c my kids have alloted spending money.
3) The coke stands give free ice water. Some people don't like the taste but we thought it was great and I only drink bottled water at home (maybe the heat made me desparate lol)
4) If it gets too hot/crowded at midday you can try two things...go back to the hotel and swim and come back later around dinner time when it starts to cool a bit or find an indoors place to eat CS's have a/c too. It was also really pleasant to have a lunch ADR or early dinner to sit and relax in the cool for awhile.
5) Many rides have shaded and/or air conditioned lines. When you are hot look for those. Philharmagic, Soarin', Buzz Lightyear, Star Tours and I'm sure plenty of others have indoor a/c lines.
6) Don't push it too far, if you must rest then rest. Don't try to "tough it out" if you are hot, tired, thirsty, meet your needs and/or those of the others with you and then continue on a few minutes behind but much more comfortable.
Good luck and have a blast!