Have just picked up this thread and of more concern to me than your lost bag was the statement “KLM cannot give the payload details as this are not available for the flight I was on”
This is basic safety information for any flight and the load sheet details how many men women and children were on board, along with details of every kilo of baggage and freight and the weight of the fuel carried. Every aircraft has a maximum takeoff weight based on aircraft type, any payload restrictions, the route being flown as well as the weather and runway conditions at the departure point. From this data a maximum takeoff weight is determined and once the fuel required for the trip is deducted that is the weight left for passengers, baggage and freight. If this is exceeded then something comes off usually starting with staff passengers then cargo then bags.
There are also routes that notoriously have huge amounts of passenger baggage such as Lagos where carriers have allowed passengers additional baggage allowance due to local needs. African and Caribbean routes are the worst for this and at times aircraft holds simply volume out. There are then flights that operate with the weight available to carry more freight and baggage but not the space to carry them.
The routing and aircraft type can also play a part especially when aircraft are at operational limits. If the en route winds are against the flight this requires more fuel leaving less weight for passengers and bags. Winds tend to be seasonal and this may account for the April to October issue with KLM.
This should also have been backed up by an electronic or manual baggage reconciliation statement detailing exactly what bags were carried by tag number and passenger name. It should also included details of any unaccompanied baggage carried such as those bags that have previously missed an earlier flight.
All of this information is mandatory and the paper copies must be retained for at least 12 months with the electronic versions for a great deal longer.
Hopefully KLM have said what they did simply because they could not care less about your missing bag rather than because the information is not available. If the latter, then KLM and its passengers have a much bigger problem than a few missing bags.