until you get a better reply...I am a RH mouse user....so RH click brings up the context menu same as you.
Some parts of your RH context menu are controlled by your File manager, eg pcmanfm.
I notice you have a move to trash...while I have delete option and no trash....personal preference.
2) You probably looking for the distro system file pathway of /usr/share/applications/mimeinfo.cache
and it contains things....relevant to what was originally installed and basically defines for a certain mime type, what desktop executable will open it
Use it .....as a template to fix your home config file
3) What you are probably hoping for....is to refresh or fix or edit your home dir version of that cache? Do you have one?
4) lets pretend you want to change what app appears at top of RH context menu....which means if you LH it will open a new default app instead of the others
so we edit or create your home file not the system file so it would read as....and check /usr/share/applications for syntax of capitals please....and /usr/local/share/applications for private builds too
what ever is listed first changes the order in the context menu get it?
5) Depending on your desktop, you can refresh it without logout using IceWM with the hot keys....I am not sure what keys you have set up to refresh LXQT but logout and login if you can not find them