The infant gets its own boarding pass regardless of when you collect this, although I believe the loop hole that used to let infants collect miles/TPs has now been closed.

The seat share bit is seemingly academic (although you obviously can't put yourself in an exit row if you have an infant with you).

As for food, BA do not officially offer meals to infants (unlike AF, CX, SQ, EK...). If there are any spare meals loaded then presumably they can get one of those, but I wouldn't have thought that requesting a child meal at check-in would be any use either as by then the food will presumably be all carted up for the flight.

Also the point about the nutritional content of the childs meals does seem to be raised from time to time and as a result we're just sticking with Swiss Jr having the regular grub like us now that he's turned 2 and it's all getting horribly expensive...