#1
In the song I'm a mess by ed sheeran I watched him play it and I know it's either half a step or a whole step down but I don't hey how he's appearing to be playing power chords but they sound like they're open chords and not just deep like power chords. I also don't understand if the tuning is just half a step down because he's eminor sounds wrong and I have to play E major for it to sound the same .

Can anyone help me figure out how to play this properly and how he gets that open sound.
#2
If you're seeing him finger an E Minor, and it sounds like an E Major, then he's in some other tuning altogether, not just dropped a half or whole step. Probably some kind of open tuning. Or, more likely, you're just not seeing and/or hearing it right.

Regarding the open sounding power chords, that's most likely just full chords played in a movable barre chord formation. For instance:

3-5-5-4-3-3 would be a G Major
7-9-9-7-7-7 would be a B Minor
x-5-7-7-6-5 would be a D Minor
#3
I think he's playing power chords but with the b and high e ringing as well sounds the same. Probably never find out until someone figures it out.
#4
Could be. That's a reasonably common technique, playing moving chords with 3 or 4 strings and letting the others ring out with constant drone notes. Makes for different chord voices, depending on what chord is being played over the drones. One of my favorites is 2-4-4-3-0-0, F#7add11, where the high E is the 7 and the B is the 11. Gorgeous chord. Used by Rush in "Hemispheres", among others, and in Dream Theater's "Along for the Ride".