Magnetic Phone Holder with 2020 M1 Macbook Pro

Based on the number of users posting here whose screens cracked "spontaneously", in my option you should not attach any such device anywhere near the screen -- not because of magnets, but because it applies stresses this computer was not designed for.

Ask the vendor of the mounting device.

This is all up to the details of the mounting adhesive used. I’d tend to expect to encounter some discoloration of the metal at removal, minimally. That may or may not clean up.

Depending on how robust the adhesive is, there might be a procedure for removal.

I’d also wonder whether the device might cause force against the lid—such as from a laptop case getting squeezed—to be focused in that same area of the lid, rather than spread across the entire surface. I’d be concerned that this pressure might then distort or even bend that part of the lid, or worse.

Again, check with the mount vendor.

Ask the vendor. It’s their product, and they know it and its design and its adhesives best.

As for my concern around the risk of dents and of glass screen breakage—whether when the Mac is compressed in a laptop case, or should that lever arm somehow get levered around—I wouldn’t want to attach nor use one of those.