second gear spins freely in one direction only (as designed i assume)
I'm also guessing that you have a FOC installed (removes reverse and all internal components). With reverse installed, the spur will spin in both directions freely. With an FOC installed, it won't, it will only spin freely in one direction.
If the OWB was slipping on the shaft, 1st gear wouldn't be engaging as that is what is required for 1st gear. The bearing spins freely when the pawl swings out and forcefully engages 2nd gear. If it was slipping, you wouldn't be moving at all, or very poorly. It wouldn't spin the shaft well enough to engage second, so it would be totally dead. (just answering above suggestions.
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It's been so long since I've had a t-maxx, I can't recall from memory, but you may be right that you can only adjust it if it's together properly.
But, you may have to take it apart anyway to go through and see if you can see anything out of place.
Also, other things that you've kind of ruled out are slipping CB shoes, slipping flywheel and a too loose setting for the spur/slipper. If any of those were an issue, you more than likely wouldn't be getting wheelies. Any of those can also cause 2nd gear to not engage, or rather engage, but when it engages, one of the three things slips, so you loose the torque to keep it engaged.
Unless I misread something, this leads me to believe it's directly related to your two speed mechanism. Take it apart and make sure it's working properly and isn't stuck. Sometimes the pawls wear/get smashed from engaging into second and get stuck in the alloy disk that they sit in.
As for the orientation of the two speed/pawl assembly, I think if your facing the back of the truck (with the front pointed away from you, you turn the shaft clockwise and the set screw to hold it on should be first with the two speed adjustment being second.
Here are a couple images from the TMaxx2.5 manual that kind of show what I'm trying to explain:
The one with the arrow or the allen key in it are both the 2 speed adjustment.
