Can you not buy a complete medicine cabinet above the vanity like in the photo gallery and install it over the toilet? May not be quite as wide as you'd like. It may not have finished sides but you can always order pieces of the finish plywood from the dealer. The dealer might even have some small pieces in stock for doing repairs. That's what I'd look at doing if it were me and it will look factory done.
The weight of a cabinet like the one you have already would be insignificant and you'd never put much weight in one either. IMHO, I can't see any issues hanging on the wall with appropriate fasteners. As long as a cabinet has a backing on it, you could always cut a section of wall panel off and put some bracing inside the wall and then put the cutout piece back in and the cabinet over it.
I love that fridge and the theater seats....