Wow, this sounds like a job for an explosives team! But seriously, a marred fiberglass shower pan can be one of the most frustrating cleaning tasks a homeowner has to face. It appears that you have exhausted all normal avenues with respect to cleaning the pan.
The following is one method you may have overlooked: Make a paste out of 2 tablespoons of turpentine and one-half cup of salt. Using rubber gloves, scrub the paste onto the stained areas and scrub with SOS or a nylon brush. Wipe up the excess paste, flood the area with fresh water, and towel dry.
If this doesn't work, there are two alternatives: 1) Replacement of the shower pan with a brand new superior product, or 2) refinishing the existing pan in- place. The disadvantage of the first alternative is that the shower walls generally must be removed with the pan. This can be very expensive and, when the walls are in excellent condition- a waste of money. Conversely, if the walls also are in poor condition it's a great time to consider a bathroom remodel.
If a remodel is not in the cards- or in the budget, then we suggest you go with the second alternative. Consult with a bona fide licensed contractor who specializes in tub and fixture refinishing. These companies offer various techniques (some patented) for refinishing existing products in place. The process, which generally will take one to two days, starts with a chemical washing of the product. Once washed, the fixture is chemically etched to create a surface that the new finish will bond to. The surrounding areas are masked off for protection and a new finish is applied with a sprayer. Most reputable companies will offer a written warranty of up to five years.