Can you get back In the Thalmor Embassy? You know when you did the quest with it?

With a horse glitch. I saw it on Youtube. But not legit-ly

I'm pretty sure the only way to get back in after Diplomatic Immunity is to use console commands. Other than that, no, I don't think so.

Nope the only way i can think of is if you find that cave that you used as an exit and try and get back on the ledge.

WIthout console commands, ^^this^^.

Although I tried it using Whirlwind Sprint, having read that was the only way to do it, and I simply could not make it happen. I'm on PC but I still wanted to sneak back in the "legit" way if I could... I just couldn't make it work.

Of course I had Ulfric modded in as a follower and was planning on using console commands to get him up on the ledge if I made it, and I used the console to make Elenwen non-essential so I could let Ulfric kill her once we did get in, so it's not like I was planning on going 100% legit anyway.

I was able to get back in one time with Shadowmere the All Terrain Horse. You have to pick the lock on the gate and kill all the guards outside the embassy. Then get on your horse, go to the left inside the courtyard, and ride straight into the fence towards the back of the embassy. IIRC there is a rock or something you can use to boost yourself up. Then just keep working back and forth until you go over the fence. It took probably 3 or 4 minutes and guards in the back are alerted to you so you do take some damage while you're trying to get over the fence.

Horse glitch, wooden plate glitch worked fine for me

You could use the horse jump glitch thing. I used it and wiped up every damn elf in there, including Elenwen. I made her my thrall and lead her to the Palace of the Kings where I chopped off her head for Ulfric. I'm sure he enjoyed it.

And they say Nords don't appreciate magic.

I got in after Diplomatic Immunity just by picking the lock at the main gate, this had never occurred before.

Not unless it's glitched or as others say console commands no you can't.