I always support ideas that make MB more customizable and editable, but such a feature would benefit so few people to invest development time into, unless this is something very easy to program. That said, I am surprised there is no "Reset to Default" button in the Hotkey options, or in all of MB from my quick search.
There is an ability to Save and Load settings from a .ini file, but I didn't find a "default Settings" .ini file in the MusicBee Program folder (it could be in another folder like AppData or MyDocuments, etc). If there isn't a default settings file included with MB to rollback to, there definitely should be.
The next best thing you can probably do though is save your settings and find some way to roll yours back to default (uninstall/reinstall Musicbee) or you may be able to download the portable version and see what settings are there.