Fiberglass isn't asbestos. You can remove it yourself. Just wear gloves so it doesn't irritate your hands/arms, slice it off above and below where you want the cabinet with a utility knife, and throw it away.

You can recess most cabinets. There might be a few things to consider. What kind of hinges does the cabinet have? If they are the "European" style like a Blum hinge, you should be ok. External hinges could pose a clearance problem. The hinge mechanism on most Blum hinge pop out a little bit to insure clearance. You can find trim or molding in myriad styles at any big box store. There are a few more important things to consider. Are you dealing with the "wet" wall? There may be many little gremlins hiding, like copper plumbing to an additional bathroom, Romex, or even the vent plumbing. You state that it is 5" deep. Most wet walls will handle that with no problem as they are thicker to allow for said plumbing, but another wall might not be that deep. Another concern is that you state that it is 23" wide. If your studs are 16" apart, you are going to have to remove a stud, put in a header, and other fun things. Remember the stud you will be removing is surely attached the the wall on the other side. Unless you have a pencil video camera, or X-ray vision, the only thing I know of doing is to put a nice sized hole in the wall & see what you are up against. Well, do you feel lucky? Hope that helps. I went through a similar exercise a few months back and had not one iota of luck. Everything had to be moved. It is an interesting DIY, not overly difficult or expensive. It is, however, a PITA to do. repurr

The new cabinet is 36" wide (triview, frameless). Yes, it's plaster, and they will have to cut a hole, but I'm hoping they know how to do that. The master bath shower plumbing is behind the wall about 20" to the left of the edge of the master bath medicine cabinet. The cabinet will go to about 17" (where the old wallpaper ends in the picture), so it may be tight. The person who did the initial measurements did look at the master bath and did some measuring, and I told her the mirror was right behind the mirror in the hall bath. They have been kind of pi**y about my questioning anything (because they TOLD me it might not work, that's supposed to cover them). We've been through a little drama over the tile repair, too. I don't know why it is that people who you hire to do work for you are not more accommodating. I don't think we have been difficult, but OTOH, it's our house and we're paying for this. I think the deal is that all these bathroom places prefer to gut and put in all new stuff. They are not really renovators.

If the gap is not more than 1-2 inches I would install the new cabinet and fill the gap with Easysand. If you need some backer, strips of foam insulation can be wedged in the gap. I had a renovation on an old house that had a medicine cabinet mounted on the wall surface. I decided to simply recess the existing cabinet (why spnd money on a new one, the old one was in good shape). After removing it and cutting through some plaster, I found the original recess had been filled with bricks (on edge) and plastered over long ago. It took about an hour to clean everything up and put the old cabinet in the recess. Some Easysand20 filled the perimeter gaps.