Fiberglass over our plastic panels?

Pictures would be VERY helpful. From what I gather, you have one sail panel with an actual crack, and all the other plastic pieces only have superficial damage. If I am wrong, please correct me, but that is what you stated. First, for the hard plastic parts, body filler will work fine in the scratches, divots and nicks. I have done it a million times. If the scratches, divots and nicks are very bad, you could use 5-minute JB Weld epoxy. Build it up in thin layers. It is a bit harder to sand than filler, so do not put on too much. But body filler will work most of the time. For scratches, divots and nicks in flexible plastic parts, I use a two-part epoxy specially made for plastic. it will fill the damaged areas and flex a bit. With hard or flexible pieces, a light, thin skim of regular filler can be used to get that perfect surface.
I am not familiar with the sail panels in your car. I do not know if they are hard or flexible plastic. Here is a short version of what I do for cracks in plastic:
Hard Plastic: I first drill a 1/16 hole at the end of the crack. This is called 'stop drilling'. It will keep the crack from spreading further. Then I widen the crack a bit and bevel both sides of the crack on the side you will see. Then I strengthen the area by applying a patch on the backside with 5-minute JB Weld epoxy and a little fiberglass cloth. Scuff the repair are with sandpaper first. Once the epoxy cures, you will have a very strong repair that will likely not flex or crack anymore. Now you just need to repair the front side. I'd use more 5-minute JB Weld epoxy like body filler. Make sure to use thin coats. I like to tape around the areas to be filled with cellophane tape (because it is thin) to try and keep the epoxy only in the desired repair area. Then sand the area down and use a light skim of regular filler to make the area perfect. I have been doing this for years with good results.
Flexible Plastics: There are lots of kinds of flexible plastics used on cars, and I would have to see what you are talking about. If the flexible piece is the kind that will melt if touched with a soldering iron, I fix the crack by stop drilling it first. Then I repair the backside of the crack with a soldering iron, and I use a plastic zip tie as filler (like using a filler rod when welding). On the front, I will probably use the epoxy for plastics mentioned above. I sometimes use the soldering iron and zip tie filler rod, but that is only in a bad crack. The soldering iron and zip tie filler rod method on the front is much harder to finish to look nice.