I have an Onyx countertop that is approx. 1/2" too long to fit between the drywall on either side of the bathroom walls. Is it possible to sand down the countertop sides with a belt sander (for example) to make it fit and if not, are there any other ways outside of taking it to a pro to cut it down? Any help is appreciated..

It probably can be cut but I've never tried. Simpler to cut out a slot in drywall, install counter top and then repair drywall. Cut slot in drywall then chisel out another 1/2" in any stud. Install counter top by lifting the far end and slide counter top into slot. Lower far end. Repair drywall. If top has back splash, and if there is a stud in the corner, you will have to inset top forward of stud then slide back to back wall.

Excellent idea! This is the area and general location (slightly lower though) it will be installed in. And the backsplash is built in to the countertop.

You got it made. Looks like you only have a few screws holding that drywall. Remove and notch those two studs 1" below and 2" above expected cabinet height. Then when you go to put your top in you can cut about a 1" slot in drywall and slip top in.

Sounds good. I figured I would just temp that side drywall in until I got this straightened out. Thanks for your help and advice. Saves me A lot of headaches.