To start off here are links I am sure you will appreciate...
Here is a link to a site you should search for "Faun"....
1. Did the Canadians use the trailer as supplied by Trumpeter, or a different variant?
No, the trailer was the 4 x 4 wheel base shorter version. Revell-Germany makes the shorter trailer in 1/72.
2. Can the variant trailers be built using most of the parts supplied? I'm not afraid of scratchbuilding with decent reference; has anybody done a trailer conversion?
The trailer in the kit is the long wheel based version. There are no other options in the kit.
Here is a scratchbuild.....
3. I presume that if one wants to do the 8x8 version, the headlights have to be changed as well as front axles. That last sounds like a lot of work, possibly having to make resin copies of the rear axles? I'm inclined to pass on that job, but if anybody's done it I'd like to know how it went.
You might be out of luck with this.
4. Loads seem infinitely variable; anything in service from the late '80s with the appropriate operator. I presume that tanks of several kinds might be hauled out of Bosnia or Kosovo; perhaps thrashed vehicles headed for a museum? Same for a Canadian truck, a Leo C1 would have been current and a Centurion likely headed out of inventory. Other ideas?
The Canadian transporters carried the Leopard C1 and other Canadian AFVs and German AFVs as can be seen by the German Gepard on the trailer in one of the link images.
5. What reference publications are recommended?
Tankograd has a book on German tank transporters. For the Canadian version the images are limited.