You dont need a key for DM (or if you do, cant remember, now you have me rethinking, you can let a mob kill you and rez inside) and for scholo, you can kill yourself and rez inside the door. For a pally with a crapload of stam and plate, this can be difficult, but what I did, was strip all the gear off, go to the lake and go under, wait until the breathing goes down and let it take you down to half life, be careful its easy to fully kill youself, and since you are not close enough to the instance, you will have to corpse run. Once you do that, hurry and go the building, and to the right of the instance and inside, there are no mobs, you just run all the way upstairs and jump from the roof to just outside the instance area, you will have to do this a few times as even with half or less health, my spirit kept regnerating my health, after about four times running up and jumping I died, and you can corpse run to the instance and walk through the door and it should let you enter.
Another option, is get a warlock who can Hellfire themselves, rez inside and pull the lever, there is a torch inside the door to open it. I actually lucked out one time, as I saw someone in there and they opened it for me.