Spain doesn't even exist in 1453. Aragon and Castile-Leon are still separate kingdoms - different laws, different customs, different traditions, different culture (heck "Aragon" itself is divided in that regard, and yet none of that impaired it holding together cohesively as the time understood the term) - without even a personal union between them.
France is divided, and England is only somewhat better - and neither are defined by some kind of "national character" at this point.
The Romans includes the 'Byzantine" period (and thus the late middle ages), and WWI - where the Ottomans held together internally fairly well (including the Arab Revolt, which hardly had the support of most of the Arab population - and the issues with the Armenians, which is hardly enough to dissolve the empire) - is centuries after the first international conflict of the Ottomans (not to mention that the Ottomans as "a perfect rotting corpse" held together for another century and half until forcibly dismembered by the Allies).
And I see that you're trying to bring in examples from the 19th and 20th century - nevermind that things have changed considerably over that period of time.
So if the Byzantines/Romans are no better off in "Greek" areas than any other they manage to claw back, if they manage to claw anything back.
Spain is in the road of Unification under Ferdinant and Isabela,same language same religion and the differences you present don't exist;minimal differences in law must exist since they were created by different circumstances and that applies in many countries today;customs that's a nice myth!even today there are different customs in different parts of the country in all European countries.customs are local,in few instances regional or they can even differ between neighbouring villages;that doesn't signify anything!