About the “Common Tongue”: With his otherwise incredible attention to detail and realism in all aspects, doesn’t seem like a pretty typically anglo-saxon AND presentist trope to have the Westerosi language be A) called the “common” tongue and B) somehow happen to be spoken fluently by so many all the way to Qarth? Why no mention of Qartheen or Asshaii languages (or accents)?

Yes and no.

GRRM has said in interviews that he had the vast majority of characters be fluent in the “Common Tongue” because A. it’s easier if all of the characters can communicate with one another, and B. he’s not an Oxford don linguist who can come up with new languages at the drop of a hat. So I think it’s more just for convenience’s sake than anything else. 

On the other hand…I think you’re also painting with too broad a brush. Planetos is a place with many languages – Valyrian and not the Common Tongue is the lingua france of Essos, and even then it’s already breaking down into separate dialects on the way to separate languages in the various Free Cities and Slaver’s Bay, there’s the Slaver’s Bay dialect with its loan words that are all that remain of Ghiscari, and so on.

So to take Qarth as an example, when Dany is greeted by the three:

The pale man with the blue lips replied in guttural Dothraki, “I am Pyat Pree, the great warlock.”

The bald man with the jewels in his nose answered in the Valyrian of the Free Cities, “I am Xaro Xhoan Daxos of the Thirteen, a merchant prince of Qarth.”

The woman in the lacquered wooden mask said in the Common Tongue of the Seven Kingdoms, “I am Quaithe of the Shadow. We come seeking dragons.”  (Dany I, ACOK)

Two out of three don’t speak to her in the Common Tongue. Pyat Pree takes a look at a small khalasar and speak to them in Dothraki, the merchant speaks to them in Valyrian because that’s what traders speak in, and the only one who uses the Common Tongue is a prophetess and shadowbinder who can see the future and who already knows who Dany is. 

As far as accents go, they definitely exist. We know that Melisandre’s speech is “rich with the accents of the east” (Davos IV, ASOS), we know that the people of the Free Cities speak Westerosi with a “lilt of the Free Cities” (Arya II, ACOK), also described as a “liquid accent.” But we also know that the different accents of Braavos, Tyrosh, Myr, Norvos, Pentos, etc. are distinct.