You can reset the default by renaming the . exe file. Sketchup changes the default exe to sketchup.exe as they expect you to only use one version. If you want SU8 as your daily use but still have su13 for when you need it, navigate to the su13 folder and change the exe to sketchup13.exe This lets you choose the one in the su8 folder as the default open with. This is a workaround and not really the right thing to do, however I haven't found any problems with doing it.

Your own subfolder of SKM material files [NOT image files 😮 ] needs to go into the main Sketchup .../Materials folder.
If you don't have FULL security rights to that folder then it might look like your subfolder has been put in there... but the dreaded 'Compatibility Files...' link button in the main Explorer Window's header-bar is a clue that something is adrift... Windows has put stuff into a Virtual store to protect you from yourself... 😒
You need to ensure that there is FULL security rights [for all] to several folders in the Sketchup folder - Materials, AND Plugins, Styles and Components - because each of those will, at some pint need files adding into them... These access-rights should also trickle down to all of these folders' contents, including any subfolders too... 😕

Allison, this is normal behavior. SketchUp can temporarily stop showing shadows and textures while you are navigating in the model to improve navigation performance. If it didn't, it would have to re-render textures and shadows many times while the camera is moving which would slow things down a whole lot.

No worries, I found animations very temperamental.