Wow!!! Well to start, the tensioner pully has nothing to do with setting the belt tension, other than applying the "tension" to the belt. There is no adjustment of the pulley bolt.
"When I push down on the tensioning idler, rotating it counterclockwise vs the big center bolt, the indicator moves from the zero position *away* from the tension position; rather, the tensioner moves relative to the indicator."
This sounds normal, as you are pushing the pulley against the tensioner force. The indicator is slaved to the inner sleeve not the outer.
Do this, loosen the big center bolt, back off the adjuster, reseat your belt. Set the pointer to the far right end of the scale. Start to wind down the adjuster, until you get to the far left end of the scale. Tighten the center bolt and button it up. If you can't reach the far end of the scale, the tensioner inner rubber component is either fully or partially detached and the assembly will have to be replaced.. Hope this helps...
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..