Hi Marcus!
I'm sorry to hear your wheel woes continue and even sorrier you suffered the bad crash. Crashes are never fun, but they can be worse on a tandem due to the sheer mass, if nothing else. They aren't pretty. I hope things start to look up for you soon!
I also have a tandem with an Arai drum/drag brake, but it is built on a freewheel hub rather than a cassette freehub.
Besides the trouble with the hub bearings, you're now dealing with two additional problems:
1) A wheel that isn't centered in the dropouts, and this really shows up at the chainstay/rim.
2) A wheel that doesn't remain in tension (spokes loosen, requiring tightening).
The wheel's rim should be centered between the hub's axle locknuts (and thus the dropouts).
Faced with the centering problem, I would first check the hub spacing to make sure everything is in correct order. As I recall, the hub has been apart several times to check on the bearing issue, and it is possible it was misassembled at some point unless this spacing issue was present from the beginning. This could explain why the rim seems to be largely centered on the hub yet is offset to one side in the stays.
If the hub spacing checks out, then it does sound like the wheel is mis-dished. As I recall, mine rides in 145mm OLN dropouts, so the wheel is pretty symmetrical with the Arai and the 6-sp freewheel. The spoke bracing angle is not very steep, but it has been strong. After I built it, it has remained true and had no trouble coping with an all-up weight of 600lb/272kg including riders, touring gear, and trailer.
Now to the loose spokes...
In my view, the continued loosening of the wheel points to uneven tension. In my experience, wheels tend to move till they reach equilibrium. If the tension is uneven, the rim will "go" where it needs to balance things out. If tension is correctly high and even, the rim will stay centered and true unless something intervenes (accident or other stressor that causes a bend). Unfortunately, wheels with uneven tension tend to be more prone to "tacoing" under high stress, and this would be really unfortunate if it happened while touring.
I would suggest checking the spoke pattern on the wheel to make sure all spokes are weaved correctly at their last crossing. That will eliminate the remote possibility it was mis-laced.
If the loosening were a one-time thing, it might be put down to a rim bent by the crash. However, to have it continue, I think one must consider uneven tension as a prime culprit. At some point I would de-tension the wheel and start from a baseline of zero tension, adding layers evenly and truing and de-stressing (removing spoke windup) after each tightening. Even without a tension gauge, you should be able to get a better result than you now have.
If the Arai's torque-reaction arm is left free so it doesn't engage the frame, you can flip it back and forth in the dropouts to get the centering correct, or even use a couple shot glasses under the rim and stacked coins under the axle locknut to make your own centering gauge on a café bar table, as I once did for a friend with wheel trouble. It worked great, and the wheel ended up centered within 1mm. A pencil laid across the brakes and held with a rubber band and the pads themselves will work for checking trueness both radially and axially. With care, a person can match the results of dedicated shop tools using these methods.
Hopefully helpful....
Best,
Dan.
