Best way to go through the course?
I have been working through one section at a time, all the topics at level 1, then level 2 etc, keeping the topics pretty much even until all are five - then moving on to the next section. Is that the recommended pattern? Or should I do all the sections at lower levels first, then go back and do them all again at the next level?
This is what duolingo recommends: https://making.duolingo.com/whats-the-best-way-to-learn-with-duolingo
I think the most important thing is not to do an entire skill in one go, but to go back to it regularly and it sounds like you've been doing that.
I would suggest you to do as it suits your learning style the best. I prefer to complete the tree to a level and then start with the next level. Sometimes, as per my mood, I complete some skills to a higher level. For e.g. right now I have 4 or 5 skills at Level 5, most others are at Level 2 or 3