I can help with a couple of these:
ORIGINAL: mangaroca
3. Why is there a Leo 2 A5 Gun Barrel in Leo 2 A6 section?
The only difference between the A5 and the A6 is that the A6 has a longer gun tube. Otherwise, they're the same tanks.
ORIGINAL: mangaroca
4. Is the second (Detailed) Gun Barrel the proper A6 one?
That's a Tamiya part. So, I would guess that it is an A6 gun tube. The A5 you're looking at is made by AFV.
??? At about 700 Euros, they don't seem cheap. Tankzone has prices in pounds sterling (yours), but AFV's are Euros. Did you factor that in?
BTW...almost all of those upgrade parts on Tankzone appear to be AFV parts that they are reselling. They've even just cut and paste images from the AFV website. Which has made it easier to hunt the maker for all of their parts.
The manufacturer I haven't been able to hunt down is who makes the metal tracks with rubber pads for their Bulldog. [>:] They look like the Bradley tracks from AFV, but I'm not sure. Tankzone's prices are just too high for me.