I have a '97 V-Rex. It doesn't get a lot of miles put on it anymore, but it probably has on the order of 18,000 total miles. Yours looks like it's seen some hard service, so that may be at least a small part of the issues you're having. Parts wear out and break with use, and the harder you use them the faster it'll happen. Mine:



Regarding your complaints, I can make a few comments.
1. Seat mesh - I too had issues with the mesh wearing. I went through two seat meshes, the second one, shown in the pic below, literally pulled apart from the tension I used. An additional complication for me was that I have an old tall-back seat and they no longer sell the mesh for it, deeming the current seat size 'close enough.' My solution was to have my sister make one out of Pet-Pruf screening material, using an old one for the pattern. The stuff is so tough, I think it will finally die when the expanding sun engulfs the earth.
2. I have the older bare aluminum sprint braces, so I can't comment on your problems there. Mine are not as stiff as I'd like, but they've been trouble-free.
3. The "cheap plastic piece" is just a nylon machine bolt. Replace it with a steel one! BTW, my bike still has the factory-original one. I covered it with 2 layers of electrical shrink-wrap tubing.
4. I have the old square-bracket seat mount, which was noted for slipping. A little sandpaper fixed it. Simple solutions are best, IMHO. If I had the Rad-Lok, I'd be using the inner tube solution. Can you just tap the stripped holes to take bigger screws?