Kadee:

The biggest gun in the arsenal when it comes to removing stains is a process called "procking", but which goes by many names depending on who you talk to.

To prock a stain out, you soak a WHTE cotton rag in bleach and squeeze it out only so that it doesn't drip. You want it to be almost at the point of dripping bleach, but not quite. You set that bleach soaked rag over the stain and cover with either a bowl or CLEAR plastic, and then tape the edges of the plastic down with masking tape or weigh them down with a length of steel chain of any sort. The idea here is just to prevent the bleach from evaporating. Leave that on for a half hour, and see if it does any good. If it seems to be taking the stain out, then leave it on for another hour and check again, and gradually increase the time between checks until you're leaving wet bleach on the stain over night.

Now, if that doesn't take the stain out, nothing will.

Try the bleach on an inconspicuous spot first, probably in the stained area to ensure that if the bleach takes the colour out of the fiberglass tub, it's in an area that was damage by the stain anyway.

You should be aware that you might not be able to replace the fiberglass tub insert with another. Some of those fiberglass tubs are too large to fit through a door way even with removing the door and the wood door moldings. Replacing them would require removing exteriorwalls like they so when a super obese person has to be taken to the hospital.

If it were me, and replacing the fiberglass tub wasn't feasible, I'd probably go with the Swanstone/Corian plexiglas panels simply because they're the least maintenance. If you're willing to seal the grout yourself, then ceramic tiling is a good option, but only if you're willing to maintain the tiling by maintaining the grout sealer. Maintaining ceramic tiling is an entire subject that you would need to learn at least a little bit about.

The only problem with the Plexiglas panels comes when you want to replace the mildewed silicone caulk on them. In that case, you can't use a steel razor to cut the old caulk off. You can buy plastic razor blades at Lee Valley to get much of the silicone off, but you'd still need to use some tools that could take the silicone off without damaging the Plexiglas, and about the only one I can think of are plastic razor blades or plastic ice scrapers for car windshields. Any metal scraping tools would likely scratch up the Plexiglas/Swanstone/Corian panels.