Engage the deck ( engine off ) then grab the tensioning arm and see if it moves further back ( making the belt tighter )
If it does , note how far further it moves till the belt becomes tight.
If you get to the end of the movement of the tensioning arm and the belt is not tight then either it is routed wrong or the belt is too long
Note the distance that the hole in the tensioning arm where the cable fits moves between belt slack & belt tight.
Then check how far the cable moves engaged to dissengaged, it should be further than the tensioning arm because it has to stretch the spring .
Rereading the entire saga I am wondering if both of the deck cable anchoring brackets are there.
There should be one on the frame & another on the deck.
If one has fallen off you would be in this position.
So what I really need is some photos.
Slide the deck out and show the belt routing
Before you do that drop the deck to the floor and take some photos of each end of the deck cable, blades on & blades off.
Also the tensioning arm cable attached & cable off belt slack & belt tight.
Please use a camera if you have one and sit it on a stand , box, pile of books etc so the photos are clear & in focus.