Before replacing my old angle drive, I never had yo mess with the belt. Now, it seems no matter what I do, the belt keeps drifting to the outside and makes noise. I'm about ready to beg someone in the Houston area who's aligned their belt to come help me as I'm getting beyond frustrated to the point that I'm considering robbing a bank to get a 4-wheel kit. Even more alarming, I'm even thinking about going to a dealer!
One thing I've noticed is that the pivot shaft sometimes wants to turn when I'm trying to torque the pivot shaft nut, an action that can change the belt positioning, so I've learned to either have someone secure the pivot shaft so it can't turn while torquing the pivot shat nut or contort myself so I can place a foot on the socket wrench on the pivot shaft while reaching around/over the tire to torque the pivot shaft nut.
You're not alone. Yes, you are correct the pivot shaft can turn when placing the final torque on the nut. For me it didn't happen at all times, but at times it did, I used an extension bar to hold while I torque the nut. The final torque on the nut will also change the belt position. This worked for me. I loosened the nut and rotated the rear wheel to see how it tracked and adjusted to position as suggested not touching either flange but slightly favoring inboard. Then I torqued the nut to 195 ft lbs. After torquing the nut, it always changed the belt track, so once I got it in the ballpark, I marked the pivot shaft at the 12 o'clock position with a silver marker and rechecked after torquing the nut. I would recheck after final torque and if necessary, loosen the nut and readjust the pivot shaft 1/4 turn at a time (counterclockwise to move the belt outboard or clockwise the move inboard) Then torque again and recheck and adjust 1/4 turn as necessary till it settled into a true track with the final torque on the nut.
It's not you it's a narrow range that shifts with torque. One important thing I noticed (it drove me nuts) is that the flanges on the rear sprocket have a chamfered edge and it will appear as though the belt is not riding on the edge when in fact it is. The belt needs to be off the edges so as to expose teeth on both sides. It's a bear!