It would be nice to have more info on the biathlon rifle.

I don't think that is a take down pin. I've seen photos from an older auction, which showed a sideways sling swivel mounted there on the other side. So it makes sense that it might be a nut that the swivel screws into from the other side.

No idea on the dingle under the trigger. A zoom in on that trigger seems to show it as coming out from the top, bending down, then re-joining at the bottom of the trigger guard. Perhaps the sling is meant to join there? Every biathlon rifle I've seen has almost a "backpack" sling, where there are two straps. So sling one from the front underneath mount to there, another from the side mount to the rear mount, and you have two straps. Maybe?