Can I stop part-way through a course, or go back to one I've finished?

Yes to both. Your progress saves as you go, so you can stop whenever you like.

When you come back, My learning shows the course under "Pick up where you left off". The course page says how many modules you've done, and the Continue button takes you to where you stopped.

Once you've finished a course you can go back to it whenever you want. Open it and press "Watch again", or choose any module from the list. Watching again doesn't change your certificate.

Some certificates run out after a set time, and the course then shows a Retake button. See "Can I retake a course, or get a certificate with a newer date?".