Can anyone tell me who this is?

Just a guess that’s probably wrong, but my first thought is Rommel.

Isn’t that Field Marshal Montgomery?

Nothing like him.

Perhaps Sepp Dietrich, but I’d expect to see a death’s head cap badge instead of the roundel shown in the OP photo.

Same nose.

t doesn’t look like any German uniform I’ve ever seen, and there’s no insignia to indicate nationality. Maybe Spanish or Italian?

Yes - little-known fact; they shared it via neutral Swiss courier. As an on-going practical joke, they’d compete to see who could return it stuffed with the most objectionable matter.

Yes.

After looking it up, I’m 98% sure it’s not Rommel. I was going by the cap of the dude standing in the background.

That is not Montgomery. He is not wearing a British uniform. Of course he could be at a fancy dress party.

Turns out it was this fellow: Hasso von Manteuffel - Wikipedia

Indeed you are correct, as verifiable at this site.

The guy in the background, what is the emblem on his cap? Looks like an X but doesn’t appear to be a swastika.