Here's your answer

I called LX this morning and it was the South African call centre.

At first I asked the agent to simply cancel the return portion of the first ticket, but she said that no changes were permitted and she couldn't turn the booking into a one-way.

Then I explained my problem and she put me on hold, and when she came back she told me that a note has been put in the booking saying that the passenger won't travel the return portion.

I asked whether I might find surprises at the airport, and she said "It shouldn't be a problem", which I suppose it's as far as she is allowed to go.

I am not particularly reassured, but at least I did my part. I'll update after travel.