OK problem 2 was just problem 1 - I'd cleaned the port twice with a dental pick but there was still a tiny bit left in a corner which was stopping it from clicking in properly. I had to use my reading glasses and a torch to find it though.
I maintain that the USB-C port is a poor design for out-and-about, non-pristine conditions. The fact that the gaps are *incredibly* thin because of that little central toungue makes it an absolute bitch to get lint out of, especially once it's been compressed