I agree that the PO likely had the tension relieved for storage.
Your "E" picture does indeed show no tension, but it also shows no blade mounted.
The tension can only be adjusted/checked with the blade mounted.
Lastly the marks on the scale for the different size blades are to indicate when the red bar spring is deflected sufficient for a given size blade. The left
(in thy pix) edge of the bar is what indicates tension. i.e. the red bar just disappears under the scale at a mark for a given blade size. Gonavy's pix shows the tension somewhere between 1/8 and 1/4. Bainin's pix is correct, but possibly misleading. The red mark in his pix is meant to show where the red bar spring SHOULD be for 1/4" blade tension.
YES running with insufficient tension WILL produce the scenario you initially described.
I would add some adhesive where the belt is skooched away from the rear edge rim.
Now about the 'groove' in the wheel. Yes both edges have a raised rim, but the front edge rim is much much shallower.(yeah I went and looked(it ain't flat across
So remount the blade, set the tension, manually rotate the lower wheel and see if the blade then tracks correctly. Do that with the backup bearing(above the table) fully retracted from the blade.
P.S. The red bar spring left edge should just disappear under the bottom end of the scale. Your "E" pix shows the red bar edge away from the end with NO tension. There is a screw near the top of the 'scale' that allows adjusting for zero tension calibration.