I had a similar scenario of myself and child on 2-4-1 points CW booking, and paid revenue for my wife. No option to have points and cash bookings on one PNR, so paid for my (non status) wife’s seat selection next to us (took an aisle and central pair - great for a family actually).

Checked in at airport no issue but my wife’s seat was moved between checking in and gate arrival, so paying for seats isn’t a magic solution. They moved her back when we explained we were flying together, because as far as they knew we were separate bookings.

I agree with the above strategy to book a middle aisle knowing someone will likely swap to help out. Crew are usually excellent in fixing these things as they’ll know who are the single travellers vs groups.