When you say swivel that clip doesn't have a swivel it's just a clip. The stainless clip and some 3-strand line - West Marine. They might be able to do the splice for a fee. Buy a small splicing fid and watch a few YouTube video's. Not that hard to do 3-strand.
And, there's really no need for a swivel on a mooring line... unless your boat is doing flips in the air! Plus, swivels are a weak point. Rope to cleat is the best.
Appropriately sized snaps are OK under tension, but they are lousy under shock. Growing up on a farm, I have seen many of them snapped by a horse, or a cow, throwing their head up when their lead is attached to a solid object.
I wouldn't use those transom hooks for any lines. Trailering tie down, yes. Use the cleats and if you need to run lines from aft cleats in an X pattern over the motors in a storm. When calm, the side cleats should be more than enough. I you want, run one line off the aft end straight back.