If I intended to play 18 holes, am I able to post a 9-hole or 10–17-hole score?

Yes, regardless of the number of holes the round is set up for, you will be able to post a score as long as 9-hole scores on a rated side have been entered.

If the round was set up for 18 holes and you attempt to post the score while leaving some hole scores blank, you will be prompted with a warning message to confirm if you want to post your score.