SUCCESS!!! I managed to get back into the Thalmor embassy, past the outer courtyard and barracks.
If you go in there, pick the gate lock and take out anybody in the courtyard. Then go into the barracks (may or may not be locked) and clean out the two soldiers and two wizards in there. The first go-through, there may be another one asleep upstairs.
After that, come back out into the courtyard and the main building door will say "requires key." Go to the right corner of the wall, where you can jump on the snow drift and see over the wall into the rest of the embassy yard. There will be a few soldiers and possibly a wizard back there. You can jump to the top of the wall, but not over it. Destruction magic and arrows will pass through though, both directions, so you can kill them from afar.
If you want to go at them in melee, take a kettle outside of the fence. From standing at the main gate, looking in, go left and around the corner. Where the wall again turns left, drop the kettle. Pick it up to eye level and then walk toward the wall. When the kettle stops at the wall and appears to have a hole in the bottom, whirlwind sprint inside.
You can then clear both buildings and the dungeons, and exit the cave, as you did during the Diplomatic Immunity quest. It seems anybody you killed then will still be dead, but anyone you didn't will remain alive. Lots of potions and gold in there, and a few decent weapons. Don't forget to steal every last one of their sweet rolls.
