thanks for your detailed report, I also doubt the source of flexure is with the trusses, first of all because there is no slop in your trusses by design and second of all because your UTA doesn't look so heavy (maybe around 2.5 Kg without finder and eyepiece?).
The simplest way to rule out the spider would be to remove the secondary mirror (and as much weight as possible) from the hub and replace it with a tiny reflecting surface. If the laser spot moves the same amount as with the heavy secondary in place, then the issue is definitely with the UTA...
Edited by TonyStar, 16 February 2017 - 06:09 PM.