arollins

You can not use the 500-miles to upgrade Int'l flights. It clearly says so on AA, however, you can use the 500-miles on your connection SEA-AA hub to LHR.
As for MCE, yes, they do open up at T24, however, dont be surprised if they are full, depending on HUB, it might be lots of elites that have already booked and are waiting on upgrades.

Here are a few things to consider, and this is just my opinion.

JFK, it will be a short flight, about 7hrs or so, with very minimal chance of sleep. Plane will be a 77W, with a 3 4 3 seating UGH, unless it swaps and you get an older 777

CLT or PHL will be about 7.5-8hr, it will be a 330 plane with 2 4 2 seating in main cabin, best to get something on the 2 side, and if lucky, you might have an empty seat next to you. These planes seats have their own IFE on the back of the headrest and are fairly comfortable in my opinion.

ORD, its about 8hrs flight, it is a mixed bag of planes, I would prefer a 767, which has a 2 3 2 seating in main cabin. No individual IFE

MIA/DFW about 8.5-9hrs on 77W, plane like JFK

LAX, about 11hr, on a 77W, or older 777

I myself have done all, except MIA and LAX on AA metal. With all things considered, being that you are GLD, so your chances of MCE at T24 will be ???, I would prefer either CLT/PHL on an airbus 330 where you can get a seat with your IFE on the 2 side, have a decent chance of sleep, maybe an empty seat next to you and minimal chances of aircraft swap.

Enjoy.