Backseat Rally Units

Yes. On the CO-DRIVER step choose AI CO-DRIVER. You still drive. The passenger is labeled RALLY BOT, and a friend's avatar can ride as that AI.

No. Account level unlocks cars. Mastery is tracked on each car and can unlock performance parts. A rewards screen can show both bars at once.

The car has to be stopped inside the start box. STOP IN THE START BOX, BRAKE TO A STOP, and MISSED THE START BOX are different prompts. Press Enter only when the panel says START STAGE, then wait for GO.

The Roblox description says 7 cars and 5 stages. A garage directory names 10 cars, and four stage names are documented: Pine Forest, Alpine Pass, Riverbend Canyon, and Sakura Pass. The fifth stage name was not confirmed.

No. Paint, rims, cosmetic tires, and fresheners are looks. Performance parts and the tire compound are the menu that changes the rating.