right consideration of yours but there are something to meditate about, and actually with my post,
I had the intention to draw your attention to a bit complcate factors and evidences, which also
happened many times in space and time to different cultures which the mainstream look as "one direct
line of identity".
Here I try to explain my point of view:
Ubaid:(bold is mine)
In North Mesopotamia the period runs only between about 5300 and 4300 BC. It is preceded by the
Halaf period and the Halaf-Ubaid Transitional period and succeeded by the Late Chalcolithic period.
Halaf culture: (bold is mine)
The Halaf culture is a prehistoric period which lasted between about 6100 BCE and 5100 BCE.
The period is a continuous development out of the earlier Pottery Neolithic and is located primarily in south-eastern
Turkey, Syria, and northern Iraq, although Halaf-influenced material is found throughout Greater Mesopotamia.
Ubaid of South are dated around 6500 BC, till about 3800 BC.
All what above you know perfectly. I just pointed out the question North and South which if carefully
examinate lead toward a new Hypothesis..... of course IMHO...
Lets go ahead.
You well evidenced the question of ""Lost civilization under Persian Gulf?""
And this, always of course IMHO, lead to have a carefully look to the all cultures which you and me have put on the table.
Now a simple fact must draw our attention on it: [bold is mine)
In South Mesopotamia the period is the earliest known period on the alluvial plain although it is likely
earlier periods exist obscured under the alluvium.
In the south it has a very long duration between about 6500 and 3800 BC when it is replaced by the Uruk period.]
In North Mesopotamia the period runs only between about 5300 and 4300 BC.
The Glossary of Landform and Geologic Terms, maintained by the United States' National Cooperative Soil Survey, defines an "alluvial plain" as "a large assemblage of fluvial landforms (braided streams, terraces, etc.,) that form low gradient, regional ramps along the flanks of mountains and extend great distances from their sources.
At this point any "ringing bell" start ?
I always have considered that in most of cultures, may be even in all, there are NOT any direct line of
development from one which were preminet to the anothers.
IT WAS ALWAYS A MIXTURE OF PEOPLE AND CULTURES WHICH LATER ON LEAD TOWARD A PREMINENCE ON LATER CULTURES
WHO THE PREVIOUS MIXTURED ONES COME IN CONTACT..... AND THEN SO ON, SO ON, TILL NOWADAYS.
The question of "Alluvian Plan" explain why on South Mesopotamia, during millennia perople well developped
and start to leave evidences of "One Culture", called Sumerian, which in turn after millennia lead toward
Akkadian etc etc.
Resuming, IMHO, there are not a "single and pure culture" but just a continuum mixing of cultures, during flow of millennia.
This mixing is due only to climatic variation conditions which during millennia pushed people to migrate,
in a sort of go and come back and created new cultures.
In more recent times (starting from abt the 24th to 23rd centuries BC - Sargon of Akkad) we see this phoenomenon happened mainly due to "geo-politicals" facts.
Just, as usual, weird toughs of mine.
Sources, just for an eagle view and starting research point for more deep details:
Ubaid Period - [en.wikipedia.org]
Halaf culture - [en.wikipedia.org]
Alluvial plan - [en.wikipedia.org]
You can not solve a problem with the same kind of thinking that you used to create it.