Sitting next to a stranger in CW is a bit different to sitting next to someone else in WT+. I would imagine that the higher class was booked as it will be more comfortable for the OP's mother-in-law.

You know, having worked for 21 years in a customer facing job you learn to know when somebody has a reasonable request (which IMO this is), or if they are taking the p**s. If I were a customer service agent and had all the details of this booking in front of me, I'd do my very best to try and get the seats allocated - 3 of them previously were. Maybe I'm just too nice.