The door did open into the corridor and the corridor is about 10' x 10' with a ceiling at about eight and a half feet high.
When Mradoric comes into range to attack depends on what kind of attack he is going to do. He can make a ranged attack, maybe even next round though it would be at disadvantage. He would have a hard time getting into position to make a melee attack because he would have to get around Corren and Leo. I imagine though that if you both used a move action once Mradoric is adjacent to Leo or Corren you could switch out one for the other but I think it would prompt an attack of opportunity against whomever is moving back to do so.
A battle map giving general locations is uploaded in the map section. Each square is five feet, but I prefer that this be used as more of a visual representation than as a battle grid. This can show the players where Mradoric and Zinna are starting out in relation to the others but it does not show all of the skeletal warriors and only shows the corridor and the room where you encountered the gas.