Queen Mab dates to Shakespeare's time, she is not connected to the Arthurian Myths.
Every actor playing Merlin at different life stages has a different eye color. Baby Merlin has dark brown eyes; teen Merlin has unnaturally bright blue ones; adult Merlin has normal blue; and elderly Merlin has brown again.
In the scene where Igraine is giving birth to Arthur, she has a completely flat stomach.
Although clothes, architecture, etc. heavily imply a barely post-Roman (5th-6th centuries AD) setting, characters still use the terms England and Normandy, which aren't recorded before the 9th century. It would be more appropiate to refer to them respectively as Britain and Armorica or variations thereof.