And Assyria state was exactly in between Levant and Mesopotamia.
Assyria originated from North Mesopotamia, expanded to the levant.
If Iraq and Syria are 'artificial' than they still don't belong to neighboring sates, they'd either be unified or further divided. But to further divide they'd need historical backing to be a state that is not 'artificial', kind of hard nowadays given that Iraq's North and South don't differ too much anymore, religious differences are quite recent.
Syria's 'heartland' would be their Western part whilst the East has a lot of closeness to Iraqi culture and traditions as well as the Iraqi dialect.
Current borders of Syria and Iraq are 100% artificial.
To some degree, but it doesn't matter much. The border with Iran is a historical divide between Persia and Mesopotamia, the border with Saudi is the historical divide between Mesopotamia and Arabia etc. Syria-Iraq border isn't exactly right as the East of Syria is closer to Iraq in many ways. Whilst the Syrian-Iraqi border might not be right historically these 2 countries still have their own culture, traditions and history which make them "real". Now a fake nation would be Kuwait and Qatar, the latter being part of Arabia, Kuwait being part of Mesopotamia through most of their history.