Sounds like you tested the cooler/radiator correctly and found no leaks.

I’d move on.

Drop the pan, break the valve body loose which will help drain half the converter.
Pull the accumulator out to drain behind it, with the valve body still loose stroke the front and rear servos to purge them of fluid.

Blow the lines. Hook everything up and add the cheapest trans fluid you can buy and run it on the stand - wheels off the ground through all the gears. Don’t drive it.

After running it for a bit wing the engine a few times to help mix the fluid in the converter.
Shut the engine down.
Remove the rear cooler line from the trans.
Place in a large bucket and have someone start the engine and place in drive. Run it until the fluid flow starts to diminish, do not run it longer than that - then shut it down.

Drop the pan and break the valve body loose again as above then reassemble.
Add your fluid and you should be good. If you are still worried perform the drain procedure one more time.

The biggest risk from coolant in the trans is it eats up the bonding agent for the clutch discs and bands.