I revised sled.txt and buggit.txt, changed right rear suspension from 11 to -1; changed the third value on centre of mass from -0.05 to 0; and changed the rolling resistance from 0.03 to 0.05 on both cars. The game no longer crashes when either car is driven (using KEVWOZEAR cheat) nor when either car appears as an opponent.
After looking at many of the SP car txt files (and valhella.txt from C1 on account of the similar 'blades on wheels' setup to the Sled) those 3 areas in the txt files stood out as anomalies. I also compared buggit.txt to grimm.txt because of their similar configurations, but they were both in SP. I wonder if Cleaver was absent from SP because another wheel-bladed car was already in the game?