Tighten up the screws on the mirror clips with your fingers, no need to use a screw driver, this also prevents the chance of dropping a screw driver onto your mirror. So tighten until the clips just barely touch the mirror, and then back them off a fraction. A good guide is make the gap big enough to slip a piece of paper into.
While you have the mirror cell out, you might want to take the mirror out of the cell and have a look at the floating pads that the mirror sits on. Mine were extremely tight and didn't float at all, meaning more pressure on the back of the mirror. Just get in there with a screw driver and a bit of a wriggle on the 3 mounting pads to loosen them up.
Originally mine also changed collimation as I moved it through altitude. I changed to heavy duty springs and screwed the collimation screws down much further to get a good deal of tension on the springs. Then also screw the locking screws in quite firmly, nothing crazy so you strip the threads mind you. Now the laser stays centre perfect no matter what position the telescope is in.
A 31 Nagler will only just frame M45 in the FOV. But there are tons of other objects were the 31 Nagler really rocks. You should try one on the Carina Neb with an OIII filter sometime.
