I would shoot for EC on that arrival day. It's a quick walk and perfect for an arrival dinner. The not recommended aspect is irrelevant to me, and I agree you don't want two afternoon-only days at MK. You mention the late EMH... since you're arriving late and doing a half-day, would you stay late? Late EMH at Epcot, despite being crowded, is really nice. You can stay past Illuminations, then tour anything you want as the crowds dwindle and then walk casually back to the hotel w no mob.
Thing is tho, is then you end up with 2 EC days on a 5-day trip. If that's good for you, fine, but your trip ends up being...
EC-HS-EC-AK-MK-MK
I don't like that... all front loaded with EC and MK back to back at the end.
If I was to plan that from scratch, I'd do the party on 12/11.
10 (1/2) - EC
11 - AK & MK party
12 - HS
13 - EC
14 - MK
If you can't move your party date, then your options are more limited. In that case I'd do...
10 (1/2) - EC
11 - MK
12 - HS
13 - AK & MK party
14 - EC
Last option is just do arrival MK. 1 long day at EC, and 3 MK (1/2 arrival, 1/2 party, and 1 full on 14th)
That would be...
10 (1/2) - MK
11 - HS
12 - EC
13 - AK & MK party
14 - MK