How to remove PVC glue from polished concrete floor.

How long is a piece of string comes to mind. There are several factoring issues. I would read the label on the tin and it will give an indication of glue coverage.

Label remover is made up of citric acid and water. That will destroy the glue and allow removal without a mark. WD40 will mark the wood permanently.

Natural Eucalyptus oil or natural Tea Tree oil will dilute the glue and enable you to rub it away without affecting the PVC.
(I would prefer not use a solvent such as mineral turps or kerosene or methylated spirits. These will all soften and remove the glue at differing rates but might cause minor swelling in the PVC material. Less likely with the methylated spirits though. You can test a small inconspicuous area first to see if there is any effect).

it can be done as most what is termed "marble" products is polished concrete
talk with a masonry business as to how it is done but from what I have seen it will be a lot of "elbow" work with out power tools

are you putting this over concrete? Is the concrete "clean" meaning your not putting it over glue. I am guessing if this is a new install it is a problem with the floor itself.

Floorbox is either held with screws or glue ... or set in the concrete floor.
Replacing floorbox can be a problem because the wires cannot be hidden inside a wall or floor without visible access.

Yes.

Best way to remove the flange is to get a inside cutting wheel for a drill. Has a cutting wheel on a shaft to use inside of opening to cut the flange loose. You can cut it as close just below floor level and buy a flange that will fit inside pipe to help with not having to bust up concrete around flange.

One other thing you maybe able to use a long hand saw to help cut the old one out to replace.

hope this will be off some help, you should be able to repair this , just take your time and get old one cut below floor level

You need moisture control if laying below ground level or on concrete anywhere as concrete is porous

IM USE A 1/4 INCH WATER LINE LOOK FOR A SINK OR WASHER CLOUSE BY AND ATACH TO THE HOT WATER LIND IF YOU HAVE LIME IN YOUR STATE IF NOT USE THE COLD WATER RUNTHE HOSE INSIDE THE CAVINET TO HIDE THE HOSE.