Tested the bag with roughly half a load (about 10 pounds). Assembly loosened on the ride home, but held together. I disassembled everything again and tried to deepen the threading in the bottom hole, gaining about one more full turn. Did not want to push it because I am afraid of puncturing the hardware inside the stem ( may go one size shorter to tighten the fit more if testing shows it becoming an issue).
Took plumbers tape and wrapped up both screws and reassembled. Fit is much better and no side to side motion. For my test run into work this morning I loaded the bag (about 20 pounds) - 15 inch laptop, lunch, two full canisters (coffee and smoothie), clothes and shoes, bike tools/lock and rain coat.
A few bumps and gravel on the way in and the adapter handled perfect, no unscrewing, no looseness. I keep my bungee wrapped between the bag handle and the handlebar just as a fail-safe, but so far very impressed.