Scrivener 2 and High Sierra

Can I safely upgrade to High Sierra and still use my current version of Scrivener?

Yup, all known bugs have been worked out at this point on Apple’s end.

Anecdotal evidence, I know, but I installed High Sierra last night and Scrivener 2 ran just fine on it, APFS and all. As long as your Mac meets the requirements, you should be fine.