Duckweed only - I doubt it and certainly you shouldn't expect much in the way of fish/plant growth.
Possibly you could have Purina Aquamax delivered to a nearby feed store - talk to the feed store.
Skretting ships and it's good.
Depends on the kind of fish and if you want them to loose weight.
Duckweed is not a complete source of nutrition even for tilapia. If it is more than 50% of their diet, they will not gain weight.
Also, keep in mind, if you are growing the duckweed in your system, you need to feed the duckweed something. It needs nutrients too. So if you are growing duckweed and fish and still want to grow veggies, you will need to provide nutrients for not only the duckweed and the fish but also something for the veggies. Normally we add some type of commercial fish feed to provide the nutrients for the fish which also provides the nutrients for our veggies.