[fullmetal]

Alternatively, is there a world in which one or more NATO countries decide to enforce a "no fly zone" over the parts of Ukraine that remain in Ukrainian control? 

Absolutely no chance.

US AWACS were previously flying in international airspace over the Black Sea to provide intelligence to AFU under the prior admin, iirc.

[fullmetal]

Alternatively, is there a world in which one or more NATO countries decide to enforce a "no fly zone" over the parts of Ukraine that remain in Ukrainian control? 

Absolutely no chance.

US AWACS were previously flying in international airspace over the Black Sea to provide intelligence to AFU under the prior admin, iirc.