I was very stiff from surgery. Specially in the first week. Getting out of bed in the morning was difficult because my upper torso'd feel like a plank.
As soon as you start feeling good and get a chance to stretch a leg and move around, the stifness in the muscles decreases. After the first week my muscles would "stir" like you do when you stretch in the morning and become more relaxed.
The reason your surgeon asks you to not lift your elbows too much and not put your arms behind your back, is your pectoral muscles have been exposed (there was an extra layer of tissue on them before) and need to restore. That's how my surgeon explained it to me.
The side that did not have complications healed very fast and I've barely had any pain. So without complications (peri) I think you should be good.
Getting the shirt off turned out easy

I didn't have to put my arms above my head. I grabbed the shirt in front, if you lean forward you can easily pull it off.