My understanding is that depending on the aircraft type, not all rows have the additional oxygen mask for the lap infant. Presumably this is what is causing the issue. CWS will know!!
Correct, it should not be a problem in CE on one level, but sometimes all 3 seats are used. It alternates by row and different aircraft have different rows, so it's quite complex. Amazingly though, the seat map in MMB is accurate based on current information, so MMB normally won't let you select the wrong family seating arrangements. The problem that can arise is if there is a late aircraft swap, then the lap parent may get moved and another passenger is moved into the vacant seat. And of course if 10 minutes later the change is cancelled, then it may then be impossible to move back.