Singapore Airlines carry every choice for every passenger in First Class.
I believe we were talking about Business Class. First Class is something else!
How about the third row in business class on board 777 that Mrs nivsy experienced "sorry your choice is no longer available"
Yes, unfortunately that can happen. It may vary on some airlines but my experience is that only one member of the cabin crew in the Business cabin (quite often the Chief Purser, or equivalent) takes the meal selections. So, it can come down to which aisle they started from first. Depending on the C Class configuration, Mrs nivsy may have been in row 3 but passenger number 25+ to be asked her choice.