I agree it was quite fun playing around with reversal RA-4. Maybe I get back at it some day again. When having no expectations at all, every discovery will be genuinely fun. And it's so cheap! I have still so much to try; for example, thiocyanate in FD; lower-contrast FD (e.g., increasing metol against HQ), or possibly two-bath FD. And SLIMT! None of these will be a silver bullet that would turn the paper into Ilfochrome, but I'm quite sure this process can be made surprisingly usable.
Anyhow, I agree that the mottling is a very weird phenomenon. I found no reason for it, nor I found any solution to it. I just noticed that when the development works correctly and achieves a paper-white Dmin, the mottling is much less severe than when there was the problem of incomplete development (cyan Dmin). But it is still there, no matter what!
Are you having good paper white with normal exposures from slides?