I’ve done a lot of swim supports. The question of what to do with a panicked swimmer is always a tough one. In general, stand off until you are able to communicate with the swimmer. A panicked swimmer will often want to be right at the cockpit, even if they first grab the stern or bow. They will often attempt to climb onto your boat. Even if you have a very stabile boat and great brace, there is a risk of capsize.
If they are in imminent danger of drowning, you are left with a decision to risk capsizing and hope that the person in the water will grab onto the boat and not you. Many lifeguarding courses teach you ways to get out of the grip of a panicked swimmer, but a PFD gives them a lot of ways to hang onto you.
If there are other kayakers with you and you can wait, having another kayaker stabilize your boat is an option.
I’ve towed many a swimmer, sometimes one on each end or two on the bow, paddling backwards with a swimmer on the bow. Towing backwards with a swimmer on the bow is a bit easier as far as effort and helps keep the swimmer’s head above water. It also allows you to keep and eye on them. It takes a lot of effort and is painfully slow, but it is safe and eventually you will get to where you need to go. If you’re confident that he swimmer is otherwise OK, then towing them while they hang onto the stern is easier. Paddling forward with them on the bow often has waves washing over their heads.
Using a tow rig alone is questionable. It does nothing to keep the swimmer above the water. A panicked swimmer is just going to use it to rapidly pull themselves to your boat right at the cockpit. Tying a paddlefloat to the end might work and give the swimmer time to calm down a bit before they get to your boat. This takes time to set up though, which the swimmer might not have if they can’t swim. We often carry rescue floats on our boats with about a 10’ line which have a great deal off floatation, but they are far too bulky to carry for normal paddling.
Carrying a swimmer on your deck depends on the boat and conditions. My boat has only moderate primary and secondary stability at best. even with mild conditions the swimmer would have to be relatively small and skilled enough at distributing their weight to get on my rear deck without putting me in the water with them.
Regarding two people in the water and one kayaker, assuming the kayaker had no means of communication, conditions would have to be very calm and the distances short before I would leave a person behind. It’s very hard to go back and look for a floating head in the waves, especially if the swimmer is being moved by the tide or current. We were taught to never leave a swimmer alone in the water, to retrieve a loose boat for instance.
For swim supports, a dedicated swim has the least chance of a panicked swimmer. Many require precertification. Triathlons have the highest incidence. People have never swam on open water and only in pools think, how hard can it be and I can bike and run. Then they get to where they can’t see the bottom, there are waves and underwater vegetation, and there are no lane markers. The ones that don’t panic in the first few hundred feet often end up swimming in circles, especially those that rely on a backstroke.