I wouldn't call warforged as having a homeland in the Mournlands, they are very much creations of the Five Nations. But even so, I always considered Shifters and Changelings as having less direct connection to Eberron lore than warforged and Kalshatar. (You can remove the former without significant impact to the setting, the latter not so much). I would consider both Changelings and shifters about as integrated into Eberron society as Dragonborn or tieflings are.
Not all of the PHB races are common. Tieflings, Aasimar, Dragonborn, etc. are still pretty rare.
Then be from the Sword Coast. Communities don't spell out every single member that lives in them, and we know from the established Forgotten Realms lore that Dragonborn have spread out in fairly small numbers across the realms. Not enough to warrant an entire settlement of Dragonborn, but enough that there can be and probably are a few in many/most of them.
You don't need an established Dragonborn settlement on the Sword Coast in order to have an integrated Dragoborn with ties to the area. There's established lore that provides for reasons that you are in or live in X on the Sword Coast. From there you can just write up the connections you want/need in your background.
Yep, and this is already established lore from the Sword Coast Adventure Guide.
They are integrated into the setting and are a fundamental part of it. That is why they are in the Eberron core book.
Right. Dragonborn and Tieflings were not PHB races when Eberron was created. That is literally the whole point of the discussion.
I will just mention that you can get from Aber to Toril by spelljammer, and so a few Dragonborn will have made the journey before the mass migration. In which case, they are likely to have ended up near where there are spaceports, such as the Sword Coast.
Why does that matter? Are there demons or devils in Eberron? If yes, then there will be Tieflings somewhere in the setting. Are there dragons in Eberron? If yes, then the same process to make them from dragon eggs elsewhere should work, so there are probably Dragonborn there as well. Perhaps not in large enough numbers to warrant a write-up in the book, but enough that they could be a PC race.
You need to read what Keith Baker has to say about this. He says there is a space for everything in Eberron, and it should be refluffed as needed. So, for example, if a player wants to play a tortle, they might have started out as a natural turtle who was mutated by the magical radiation of the Mournland.
Or they were in Embarrassing all alomg just like the Drafonborn, but earlier descriptions just folded them all under the label "Lizaedfolk". Easy peasey.
In the 3e Eberron book it has a section for Other Races. Now it talks about monsters as PCs there, but if you can be a monstrous race not mentioned in the books, you can be a Tiefling or Dragonborn. Especially since the 3e monster books contained numerous Planetouched races. You can be a Wispling or Maeluth, but not a Tiefling or Aasimar?
In Baker's Exploring Eberron (good book, recommended by me) he has this to say about aasimar:
He then goes on to add four additional subtypes of aasimar as well.
