I'm an iMovie user like yourself, so not speaking for Apple here.
When you get a message that iMovie cannot open the .mov file, that means that there is some codec in the .mov bucket rendered by your camera that is incompatible with iMovie. There is nothing "pathetic" about that. If you want to use iMovie it is your responsibility to record in a compatible format or convert the .mov clip to Mp4/AAC or other format that iMovie likes. Photos is a different application, so just because the .mov file may play there doesn't mean that it will work in iMovie that needs to unpack it for editing. I have found that iMovie can be more sensitive to codec variations than other Apple apps. For example, not all Mp4 formats play in iMovie. It must be Mp4/AAC.
You may want to try converting your .mov clip to H.264, Mp4/AAC with the free download, Handbrake. Do that even if the codec already is indicated to be Mp4/AAC. Re-rendering often cures the issue.
You can get Handbrake here:
https://handbrake.fr/
A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to complete. Then save and import the converted clip into iMovie.
Or sometimes you can cure the incompatibility by Control-clicking on the clip on your desktop and selecting Encode Selected Video Files from the drop down menu. That will render a M4v file that should work in iMovie.
-- Rich