Detached screw mounts - can I re-attach them

If I’m not mistaken those are standoff points that are soldered directly to the board which would require micro soldering tools to fix but you should be fine to leave them as they are. Just make sure you’ve retrieved any loose bits to ensure no damage is done after the repair is completed. I’ve had lots of phones come in without the shields or screws present and they work perfectly fine. They’re just there to make sure the connects don’t disconnect on impact but I have yet to see them disconnect from an impact.