"...threaded hole underside of the buttstock..." Yep. Should be replace the mag cap and screw in the rear stud.
Been using a Sterling sling on my real 870 for eons. Daft, but it works.
"...rifle sights..." Certainly possible, but another barrel will be less fuss. The receiver drilled and tapped? Epp's wants $50 plus the cost of the sights if there's no D&T. $75 if it's required.