Hussar
Legend
Nope. When you squeeze, you squeeze the whole time. A readied action cannot include movement and attack. It can only include one of those.They couldn't use an opportunity attack if they did get one. They readied an action which includes movement and attack, so the squeezing has ended and the person entering the room is attacked with the reaction.
(bold mine)Rules for Ready said:First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it. Examples include "If the cultist steps on the trapdoor, I'll pull the lever that opens it," and "If the goblin steps next to me, I move away."
So, no, your readied action could be a move or it could be an attack. So, if you attack, you're still squeezing, so it's at disadvantage and the PC's move up and block you in the corner, forcing you to squeeze the whole time. If you move (and that movement has to be specified when you take the ready action) you potentially eat an AO but also must take that move, meaning that you abandon the cover.