Long post incoming, beware!
Not that it matters to the subject at question - but I will I die on the hill that the inhabitants of Dorwinion were a mixture of the Nandor and Avari...and going further than that - I'm about 30-ish% sure that Bladorthin was a Sindarin king ruling over the more primitive Elves in the region.
In fact, I think that Bladorthin was one of the Sindarin adventurers (perhaps he was one of the East Sindar living east of Doriath, a folk who raised pigs, sheep and cattle as well as engaging in the cultivation of numerous agricultural goods, such as wheat and barley) -- after all, these people lived on wide open plains east of Doriath (most likely Estolad), and only nominally accepted Thingol's authority.
So, after the end of the First Age and the beginning of the Second Age, I imagine Bladorthin and his followers didn't look fondly on mingling with the Noldor - so they embarked eastward, along with Amdir and Oropher/Thranduil.
However, given the fact that Amdir, Oropher and Thranduil were used to living in the great forests of Doriath, it is only natural that they picked the woods of Lothlorien and Greenwood the Great, respectively.
But...what about people like Bladorthin (PSA! - of course, this is all pure speculation) - whose name is most certainly Elvish (probably 'Noldorin' or 'Gnomish' - a precursor to 'Sindarin'). If he really is one of the East Sindar, it would make sense that he would be avoiding woods - much preferring wide open fields and prairies.
Even the name 'Bladorthin' (while originally used as a name for Gandalf in the writing of The Hobbit), while not completely clear, does give us some hints - and to quote John D. Rateliff, the author of
The History of The Hobbit:
John D. Rateliff, noting that Tolkien never explained the meaning of Bladorthin, identifies the name as "clearly Gnomish (or perhaps Noldorin)".
The Gnomish element
blador "probably applies to wide open country" (cf. Bladorinand, an early name of Beleriand), whereas the element
-thin likely has the meaning of "grey" (as in Thingol). This would give the translation "the Grey Country", "Grey Plains Fay", or "Grey Master of the Plains"
...I mean - what better name for a Sinda farmer and cattle-raiser in the wide open fields of Rhun (or close enough to it), than
Grey Plains Fay or
Grey Master of the Plains. (The more astute of you might see the blatant connection with all these 'Grey' references - after all, the word 'Sindar' literally means 'Grey-folk' in Quenya, but I digress.)
Which all leads me to my final argument against the Mannish (and especially Gondorian) interpretation of the inhabitants of Dorwinion: and it is the simple fact that when Tolkien was writing
The Hobbit, THERE WAS NO GONDOR, much less the idea that Gondor spread so far into the east.
Don't get me wrong - Tolkien have might have retconned the issue afterwards, but I can find no evidence that he ever did such thing (more accurately, that he ever retconned the inhabitants of Dorwinion being subject to or colonized by Gondorians - and moreover, Gondor's control over Rhun was pretty tenuous and didn't last for that long: think about some backwater province at the far end of a vast, real-world empire, for example).
And before I end my rant, I'll just point to a pretty obscure passage from
Parma Eldalamberon 17, p. 54: (mind you, this text is from a pretty late stage of Tolkien's writings)
Quote:
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Even so they were under 'Sindarin' rule or influence. Galadriel was Noldorin and Celeborn Sindarin (a kinsman of Thingol). Thranduil and Legolas are both Sindarin names. So are Haldir and Orophin in Lórien; whereas Rúmil is actually a 'learned' Quenya-name. In the Hobbit all names are translated except Galion (the Butler), Esgaroth and Dorwinion. Galion and Esgaroth are not Sindarin (though perhaps 'Sindarized' in shape) or are not recorded in Sindarin; but Dorwinion is Sindarin meaning "Young-land country" or Land of Gwinion. (It was probably far south down the River Running, and its Sindarin name a testimony to the spread of Sindarin: in this case expectable since the cultivation of vines was not known originally to the Nandor or Avari.)
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- emphasis mine.
Mind you - the highlighted passage isn't exactly the smoking gun - but I have a strong hunch that the Sindar have helped the Nandor/Avari living in the area of the Sea of Rhun, if not in Dorwinion itself.