steam's cache of installed/downloaded/updates to games can get corrupted and cause this. try the 'clear download cache' option in settings and then restart steam and try to uninstall a game again.

What happens when you try and uninstall the games through the Steam app?

If nothing, close steam, and reopen it through a terminal, and try again, that way you should see log messages in the terminal of error Steam finds

I don't remember where its actual folder is, but Steam creates a link to it in your home folder. ~/.steam/steam should take you there. In there, games are installed in the steamapps folder. If all else fails, you can delete the game's folder from there.

This is assuming the game in question is installed on the same drive as Steam.

It just works for me using the steam .deb on LMDE. You click delete and it's gone.