It’s a conceptual nightmare!

I ran into something similar where the main Syncthing interface was limiting a folder to “Send Only” with that same “locked to read only” message. In my case I was able to work around it by going to the Web GUI and there I was able to set the folder in question to “Send & Receive”. I don’t know if it will apply to your situation, but you might give that a shot if you haven’t already.