Is it possible to setup a test XF install, import data, create a style, make changes, etc. - then, when it's all ready to go, blank the data and re-import and "go live"?
Or would I need to reinstall and re-do any changes I'd made?
I tried this and did it three times. The issue I found was that it tripled things like likes, post counts, etc. There's probably a way to stop this, but it was too late for me.
Install XenForo.
Create the style.
Set up any options, trophies, media BB Code, etc. (even groups and forums if you wish, as I did).
Install any add-ons.
Take a backup and keep it as your clean master installation.
Then you can import, test, reload your master backup and import again as many times as you wish until you are ready to do it for the final time.