I saw another source citing the 1200 model as using easton tubing, might be that Trek changed tubing suppliers for the 1200 throughout the years?
Reason for all the bonding and rivets is that it allowed for construction of the frame without applying any welding heat. The frame could then be constructed from tube that were heat-treated prior to construction, no need to heat-treat the entire frame afterwards. For same reason, in early 90's Raleigh USA briefly built a technium model frame from strait gauge Reynolds 753 tubes, getting all the strength benefits of the 753 without need for highly skilled framebuilders capable of silver brazing 753.
Bonded lug bikes I remember that were notorious for failing joints were the the early carbon fiber tube specialized Allez frames, few of those early Allez frames would survive more than about a year before needing replacement.