The advantages I see from having a swivel are two fold: (1) The freedom of movement the decoy has on the water is greater than what you’d see with the line run through the keel. It’s nice on the calm days to take advantage of every little bit of wind, and in the current/gusty stuff you’ll have less issues with the decoys not riding well due to an unbalanced anchor. (2) Deployment/Retrival can be less frustrating because the swivel has less friction on it than a keel-through setup, I use 400# mono on my rigs and it’s nearly impossible to get them tangled up. In the rare chance it happens a few rigorous shakes and all is well again. It’s pretty cheap to rig up a swivel setup by using a split-ring to attach them to the keel. I’ve been running mine this way for the past four seasons with good results! Weights will depend on your water depth and bottom composition, with a 36” drop I can’t imagine you’d need more than 4 oz. I ran 6’ drops with 8 oz hunting the rivers back east and the TX coast on 4’ drops with 5 oz (pyramid molds on all).