I don’t know if those dust covers would work or not. Personally I would be too concerned that they will interfere with the springs to try them.
As for your choice in diff fluid I don’t know anyone who runs a heavier rear then front in an R/C. I am sure plenty have tested it. I believe the reason behind that is to keep the rear end from wanting to kick out. They keep the AWD front biased so you can just point and shoot. 5k front 7k center 3k rear is what I want to try.