I understand your predicament in keep or release.
I rescue and rehab ducks including muscovy ducklings in NE Florida. I don't rehab wild or feral ducks as house ducks or pets, though. I don't handle them unless necessary and they basically only have ducky company
When it is time to release I have to find a pond or lake that is entirely on private property as release to the wild is illegal in Florida.
That you are planning to release these ducklings back into the wild of your grandfather's back yard is fine.
As for your question about identifying them in the future, legbands are the only durable and safe way of identifying birds in the wild.
As your ducks have had some human contact and care, as written above, they will not be fully wild. I think you should offer food and water daily, away from your house in your grandfather's garden. They can then return for food if they want it. But muscovy are feral in FLorida and neighboring states [and elsewhere I have read on NExtDoor but not seen]. They have not been domesticated for thousands of years like pekins. They can fly and they do have survival instincts. If you let them go, they will do as well as any other muscovy out there in the big world.