Hopefully some can give you direct access. With QoLmod its only possible with little complicated way Export car with debug menu. Modify mileage. Import car. Delete old car with debug menu.
Two options to remove MelonLoader 1) Run melonloader installer again. (there is uninstall button.) 2) Remove the file version.dll file from the Game's Installation Folder.
Steam version of game seems works fine with melon loader. Can you start the game normalliy from steam without melonloader? Do you know how remove melonloader? First only install melonloader and test, game works from steam normally. Then add mod.