The Dutch law is for 80% EU law so how full sovereign would Ukraine be then?

The Netherlands is free to exit the EU at any time. It has joined it voluntarily.

"Whatever the result, the long-term containment of Russia is clearly in our interest."

Who is ours?

Any European with a functioning brain. Russia is an existential threat to the Ukrainian people. Muscovy is an imprisoner and destroyer of nations and peoples. It is currently waging a genocidal war on one of its European neighbors and it has clear intentions to restore its dominance of Eastern Europe.