You don't need to pass every checkpoint if you go in reverse. The game has a mechanic where it assumes you are completing the race as intended (forwards, instead of backwards), and if you happened to miss a checkpoint and get the one after it, it will mark the checkpoint you missed as completed.

So, theoretically, you don't need to get ALL the checkpoints. But you do need to complete the race.

I do have a question, as I have to learn this soon. After the completion of the first race, in the current WR, the races afterwards seem to automatically complete. How is this accomplished? Is this because you beat the 1st race backwards? Is there an exploit or glitch required to do this?

The developers were lazy and did not make the checkpoint system reset every time a race finishes; because the checkpoint system remains the same (it thinks the race is complete) whenever you start a new race, you instantly win. No exploits needed, just complete any race.