Art of Rally

Can we just quit the races once started as in forfeit, or terminal damage?

I didn’t try that, but probably terminal damage would count since it counts in the 1000 stages achievement.

my understanding is as long as you finish the season with at least one restart, you get the bonus liveries, so you can use them as long as you leave at least one.

You definitely don't need to complete the season without using any restarts. I think I had 2 different rallies where I used one restart and I've completed the game and got the achievement.

I was able to get this with some thirds, the rest were all podiums at least.

I believe the game tells you you only need one of the five restarts you’re given: the first time I restarted it popped a warning that told me at least one restart was required for season finish to gain the bonus.

Must you win all stages?

Just finished the career mode, I won all stages bar two (both 2nd). Redid one of them to get first. Every stage has, at most, two restarts used (accidental thumb presses), and it's not popped :(

Can confirm that you can use restarts, just not all of them. I used some on the group 2 rallies.