........ So my question would be - is there any way to bring them back after they're gone?
Yes, but you'll have to do some work when it's "gone".
Like i told you, the files and both folders ("movies" and "UISounds") in the UIData folder are taken as default from rFactor.
The UI with the additional buttons, you want to have as default, is a customized UI from the HRC mod.
Now move to the rfm-folder and open the *.rfm-file from the mod.
You'll find some lines like:
OSCFile=XXXXXX.OSC
OptionsDir=UIDATA\XXXXXX
Take the whole content of the folder that is set at "OptionsDir=" including the *.OSC file and copy it into a new folder (call it "my favorite UI" for example).
Now rename the *.OSC-file into UI.OSC and copy the whole content of the "my favorite UI"-folder into the UIData-folder of the game and overwrite the existing files (you can backup the original ones, to keep them available fast).
Now every mod without an own UI will use it.
When it is gone again, you will have to open the *.rfm-file of the actually loaded mod and look in which folder the UI from the mod is and what the name (if there is another name set at "OSCFile=") of the *.OSC-file is.
If necessary, rename the *.OSC-file in your "my favorite UI"-folder and copy the whole content into the folder that is set at "OptionsDir=" in the *.rfm-file of the mod.
I hope i didn't forget something, but normally it should work.