Thanks so much again for your thorough suggestions! Now my immediate concern is:
I'm expecting a callback soon from the plumber, and it's possible he is handy enough to tackle these two types of wall-panels alongside his plumbing installation.... The problem is, that he tends to interrupt as he's short on time. So how can I concisely ask him if he feels capable of including sheetrock/panel installation along with his plumbing-installation? If i don't propose it quickly & with the right lingo, i'll be interrupted or misunderstood, if you know what i mean. He does know of a tile guy, but i doubt that such a guy would bother with a job as tiny as this. I guess I can try to find the right kind of handyman for this, which will also be very hard and dragged out, unless he's willing to coordinate his timing with the plumber's timing.
P.S. I just found on amazon this which looks interesting. I'm pressed for time, or i'd ask HomeDepot if they have something similar & more affordable:
Duzzy 78 Inch Flexible Quarter Round Molding, Silicone Rubber Peel and Stick Shower Water Barrier Suitable for the Shower Corner or Kitchen Countertop Sink and Cooktop