As I see it, you have two problems here.

1) Adding the infant will allow BA to link the bookings (this is the exception where they still allow it) so the existing booking can be brought together with the baby. You will then be assigned seats probably all in row 14 or 20.

2) Your brother in law will be on a seperate PNR so it's going to be a case of waiting to T-24hrs before he can be allocated a seat.

As an aside, having flown with our son, the first time at 10 weeks old and most recently long haul at 15 months old, I think the lower deck is probably better. It's more roomy, you have more storage space and that front cabin is very pleasant. I also think you get less of a DYKWIA/more child-friendly quotient on the main deck Our son's not loud (yet, fortunately etc etc) but I can imagine sound travelling that much worse on the upper deck...