No need to worry about oxygen mask provision for the infants. The First seats are designed to allow for "buddy dining" during flight, so two people sat at each seat area. Taking that into account, then there will be at least two oxygen masks per seat area, if not 3 per seat.

As for the use of the Birtax seat, make sure you request your preferred seating type for the infant as soon as you board. As you have infants, I would hope you would be boarded first. So make sure you mention it to the First cabin crew as soon as the hello's are done. Seat V Cot are allocated on a first come, first serve basis. The statement from the representative over the phone sounds a little bit flippant. There are a set amount of seats and cots loaded per aircraft as as said, are allocated on a first come, first serve basis and once one type is gone, then they are gone. There is not an unlimited supply, so make your choice known ASAP.

If your stroller/push-chair does end up being checked into the hold, you will not see it till you get to baggage claim at JNB. If the cabin crew are unable to stow it onboard because it is too large or there is no space due to wheelchairs, then it will have to go in the hold. You are able to borrow strollers from the Flight Connections centre at T5 to use during your time in the terminal and can then be left at the gate when you board your JNB flight. They are not amazing ones with all the whistles and flashing lights, but will be suffice for a few hours in an airport terminal.