I share your concern with the return line. I really want to avoid worrying about overfilling a tank as you pointed out. Still thinking about an exact solution... I agree a float switch will do it, good idea.I like #3, but there is a concern.
The return will be a little unpredictable if you just Y it and unless you add a diverter valve to put it back into the tank you're pulling from, or just return to the primary and have some electronics to shut the secondary tank pump off when the primary tank is full. If not and you have 3/4 tank in both and forget and leave the secondary on, you could overfill the primary to the point it's burping out. A simple float switch and relay can handle this.
But having 2 jockey pumps, one from each tank to the swirl pot is my preference once you eliminate the overfilling issue.
I'll be welding my own swirl pot, so I'll just include a 5th port and level sender and eliminate the check valves. For that matter, I'm using Bosch 044 pumps and I'm pretty sure they have an integral check valve.
Do you have a part # for the 044 pumps? The bosch pumps I am finding are twice as expensive as the Walbro pump I found.
An integrated check valve would be nice. Should also work to keep one pumps from filling the other tank other than the swirl pot.