I'd just use Danzig for everyone. Conquer Danzig -> release as vassal (Danzig tag), diplo annex 10 years later, it won't have nationalism and you can essentially westernize in 145X
Why isn't Rome included in the cities?
Isn't this quite ahistorical? Especially for the Ottomans, sultans were murdered for trying to modernize the state and the army. The Ottomans being able to simply conquer their way to Westernization seems a bit... cheap?
Where is it stated?Well, you won't be able to westernize in 145X because there's a minimum tech needed to enact the decision as well. Probably at least Admin 10.
The game is designed to follow history but players should remain free do change it. Nobody forces you activate this national decision. But players willing to will be able to do so.