Well, manipulation worked. Apparently the RNG was already set up correctly. At least enough for my script to find some possibilities when I left my last upload.
50540 cash after race 15. Enough to ditch a few money bags I picked up and still get Mega.
46140 cash after race 14, the difference between Black and Silver would mean an earlier Mega.
42090 cash after race 13. Skip Red and Skinny's, get Black, and we might have Mega then!
It does feel right to get Red for r2, Silver for r4, Skinny's for r5, and Mega for r16. Black for r3 and Mega for r14 is apparently possible, but then it's Black+Standard vs. Silver+Skinny's for r5 to r13, among other differences.
... Yeesh, the lag. It's the reason I went into that side spot for the extra life in race 13. Not taking it meant lag. It might be that taking every money bag I did avoids lag, so who knows?