Originally Posted by
corporate-wage-slave
...then the MiL's revised booking could be cancelled and joined back into the main booking.
Nope - there's no ability to 'undo' a split regardless of availability changes or however 'good' the agent might be. They may consider seating MiL for free as goodwill but she'll remain in the new split PNR. The only way the whole party would be in the same PNR after a split is a complete cancel/rebook of all PNRs involved.