Right unfortunately we can't turn back the clock and mark the crank, but if you can remember exactly where that was, then I would try fitting the belt there.
The belt will not stretch much at all, the belts are internally strengthened and the danger is that they will break with age but they do not stretch.
Just to clarify it did start ok the first time, but 5 minutes later was a problem. There is an issue with the crank pulley not moving, but at this stage and if you can be sure you know exactly where the belt was I would put it back in that position. Then turn the engine slowly by hand to ensure that nothing will touch and try it.
As long as the valves do not touch when the engine is turned over you have nothing to loose. You now know how the tensioner should be fitted and this will rule another problem out.
I am concerned that somehow the flywheel timing pin is not locating correctly and as you have taken the belt off and replaced it so many times now, without success, I think going back to the first position would be good.
If it does start this may discount the reluctor problem, but if it doesn't now start in the original position then it could still be a damaged reluctor.