Belt adjustment for a 44A mower deck on a 1250

[sbuck]

I've recently picked up a Cub Cadet 1250 with a 44A mower deck.

I'm trying to troubleshoot why it's tearing up the PTO belt.

Based on this thread - I believe the correct belt is IH-117465-C1 which when I look it up, that's V-3/8 X 79.

I tried to order that specific belt but the company I ordered from took forever to ship, so I eventually cancelled my order and got this off-brand belt - also a V-3/8 X 79.

I hooked everything up and gave it a test run. It cut some low grass for a couple minutes w/o issue, so I moved on to some really overgrown grass (about 1.5 foot tall; deck was as high as it could go) and it did a row before the belt started smoking and I turned it off.

The belt is in tact, but it got chewed up around the edges and was clearly about to fail.

To help troubleshoot this, here's some info:

Mule/belt tension:
I can not get the belt to tension according to the instructions in the manual/the diagram on the front of the mule. When I push or pull the right pulley (which I understand is used to adjust the belt) as far back or forward as it goes, it has no impact on the spring-loaded pulley on the left. It's almost like the belt is too big.

Here is a video I took showing this, as well as the routing of the belt:
https://www.youtube.com/watch?v=DC_16tFq9q8

(You'll note in the video the deck is a bit worse for the wear with rusted out spots. I will patch that up once I confirm I can get it operational)

Spindles:
I greased all 3 spindles on the deck. They all appear to spin fine.
The middle spindle is the only one with some play up and down and feels springy. Not sure if it's supposed to be this way or the spindle is toast?

The track for the belt on the middle spindle was also bent a bit in one part that I straightened out. All three spindles do have some rust/wear that I could do a better job cleaning up if you suspect that's the problem.

Thanks in advance for any insight!