How can I rotate a slo-mo video?
Hey it turns out if you're using Mojave you can use Quick Look to rotate a video right in the Finder, then import back to Photos and still have Slo-Mo!

Select your video in Photos and from the menu bar choose:

File > Export > Export Unmodified Original For 1 Video...

Make sure "Export IPTC as XMP" is checked. Save your video to a folder. There will be two files, one for the video and one .xmp for the metadata.

In the finder, select the video file and hit space to bring up a Quick Look window. At the middle right of the title bar there should be a little button with a rectangle and a curved arrow for "rotate counter-clockwise". Click it, or option-click for clockwise.

The video will rotate. If it's large, it may show an "exporting" notice and take a moment. When it's done, use Quick Look again to verify that it has rotated. It'll play without any slo-mo, but that's alright.

Grab the folder containing both the video and the .xmp and drag it back into Photos. Photos will import the rotated video and give it all the right metadata, including slo-mo! Now you can delete the original un-rotated version.

I don't know why this works using Quick Look to rotate but not Quicktime Player. I don't know why they didn't just build rotating videos right into Photos. But at least it works!