Hello Jake, I will give myself a try by replying first.
As far as I know, the demo is not the latest version. If
"latest" is referring to the very latest versions, being the experimental builds (for testing purposes - they do work fine overall but are not "production-safe").
When you have a license, you can indeed select different versions and switch if necessary. Downloading all versions would be quite superfluous. There is the FAQs page:
https://home.otoy.com/render/octane-render/faqs and you can also join the official Octane Discord server as there is an active community.
If you are interested, I have wrote some articles about Octane (the "core" renderer and Standalone, not specific to any 3D DCC):
https://www.elsksa.me/octane.
I would be curious to hear any feedback coming from someone new to Octane, if there are any aspects that you think should be covered, for instance.