Forget about thermal welding. That works for single-layer, linear polyethylene hulls, not three-layer poly hulls. On a triple-layer poly hull and heat sufficient to melt the outer solid stratum of PE (which is what you are doing with thermal welding) will also melt and collapse the inner “foam” core of the hull material.
I very much doubt that you will be able to pop out that dent either. Usually dents of that type have already caused some crumpling of the foam core that will not be reversible. If you really want to get rid of the dent, your best bet is probably to fill it in with West Systems G Flex epoxy thickened with some silica powder. In order to get a decent bond to polyethylene, you will first need to pretreat the hull material by flame oxidizing it with a handheld propane torch. Instructions for how to do this are included with the G Flex epoxy kits. Once the depression is filled in (overfilled slightly) and the epoxy has cured, the surface can be sanded fair and flush with the surrounding hull. Then cover the epoxy with some spray paint that provides a decent color match.
To replace the seats, I would probably just use the seat hanger trusses sold by Eds Canoe. Ed’s Canoe also makes seats with a variety of spacings between the front and rear seat frames. If you want to use the same holes in the gunwales as the current seats use, you will need to get seats with the correct frame spacing. The same applies to seat hangers. But the spacing for the seat hangers and the seat frames will not be the same, because the seat hangers are oriented at an angle to the keel line.
If you are measuring for truss-type seat hangers, use the hole-to-hole distance between the gunwale holes. But that distance will be too long for the center-to-center seat frame spacing. To get the correct distance for the seat frame spacing, take a yardstick or straight edge and lay it transversely across the gunwales so that it is aligned with the gunwale holes for the seat that is closest to the center of the boat. Now measure from the other hole to the center of the straight edge. That will be the distance you want between the centers of the seat frames. I would take those measurements and call Ed or his wife to make sure he has what you need and are ordering the correct parts. Ed’s Canoe does make “Old Town replacement seats” but if I remember correctly, Old Town used a different hole spacing for their plastic seats than what they used for their wooden seat frames.
If you install new seats, I would not cut the frames so that the ends are jammed up against the hull. That can promote hull deformation or even cracking and with a decent seat hanger, it is not required. Leave a small gap between the ends of the seat frames and the hull on each side.