would iMovie 9 or 10 work in High Sierra 10.13.6?

iMovie 9.0.9 works with El Capitan or earlier. With later operating systems, like High Sierra, it has lots of issues -- freezing up all the time, black still images, fuzzy vids, etc. It might work to a limited extent, but it's a struggle. Plus, it is not sold or supported by Apple any longer. If you have a second computer that is running El Capitan or earlier you can successfully use iMovie 9 there. Or, you can create a boot disk on an external drive, and run El Capitan or earlier operating system on it for use with iMovie 9.

I've not personally created a boot drive, but here are some links (none of which I have tried) that show how it can be done for different operating systems:

https://www.macworld.com/article/2367748/os-x/how-to-make-a-bootable-os-x-10-10- yosemite-install-drive.html

http://osxdaily.com/2012/02/17/make-bootable-os-x-10-8-mountain-lion-usb-install -drive/

http://osxdaily.com/2015/09/30/create-os-x-el-capitan-boot-install-drive/

iMovie 10.1.9 is the current version. It was designed for High Sierra. iMovie 10 has a very different user interface and structure than iMovie 9, so there is a bit of a learning curve and that might account for some of the negative reviews. Once you get familiar with its features it is pretty easy to use. It does lack a few features that iMovie 9 had, but on the whole it has all the basic functions necessary to create artful movies.

-- Rich

iMovie 10.1.9 (latest version) works fine in High Sierra 10.13.6. iMovie 9 is pretty much dysfunctional in High Sierra.

-- Rich