Parker,
Good observation re the bridge-slip issue. I think all of those problems are because of the relatively short neck block, which allows the body to torque under string tension. I'm not saying that neck comes loose from the neck-block, I'm saying that you have to treat the symptoms of the body torque by reseting the neck. Same thing happens with Star Fire II/III guitars, SF predecessors and M65/M75 type guitars.
Every Duaner I've ever seen either needed a neck re-set or already had one. My Red '69-'70 Mahogany DE400 came with a reset neck and a fret job. I also owned a blond DE500 with mini-buckers in the early '90's. It needed a neck set and was almost unplayable, at least with the Bigsby in place. I remember playing a friend's DE500 that he inherited from his Uncle (a 2-owner guitar, see pic below). It needed a neck set.
Back to the bridge-slip issue. The truth is, Duaner necks tend to fall into the the cutaway because the neck block is 1) short and 2) almost nonexistent on the 'cut' side. Again, as a generalization, the neck dive is less likely to happen with the approximately 3 3/8" tall neck-block on an X175, compared to the 'short-stack' Duaner neck block. The neck-movement in turn makes it look like the neck-to-body angle is out of alignment from the get-go. One can fix that by moving the Bigsby hinge over to the bass side of the of body, but you'll have to dowel the screw-holes and strap-pin, then start over.
Here are two pics of my DE400, one with mini-bucks and an archtop wooden bridge (probably a Hagstrom/Guild), one with DeArmonds next to a friend's DE500 at a Dallas Vintage guitar show around 2010. That D-55 was a great Tacoma-made guitar. I bought it from Scratch, way back when. The SF V belongs to a friend. I still have the early '66 SF I bass, but the fretless JS2 is gone.