I ended up rewatching Detention again while waiting for The Metaphysical Engine to work properly, and made notes - Matteusz said he was "afraid of who you are - what you are capable of - what you might do". Also confirmed that Charlie said "I love you but I'll lose you" - and I think he was holding the rock at the time, so that looks like a guarantee. Of course, there are many ways of losing someone, but given the body count at Coal Hill I'm not optimistic about both surviving.