I don't think there was ever historical Noah. Whether there was one or not, and whether there was a Flood or not, it is not the point is it?
Your OP post is about the age of these patriarchs, of both before and after the Flood.
I agreed with you on the issue that long-lived (200+) biblical figures are not possible. I personally believe that it is simply mythological representation, if not errors, that the ages were exaggerated. That's the nature of myths, exaggeration of stories, whether they be written or oral traditions.
It is quite possible for people to live to 120+, but they are rare cases.
There are no evidences to support that people were living over 200 years. But the average ages in the 3rd millennium BC were definitely shorter than the present, in which case, Noah was said to live on the 2nd half of the 3rd millennium.
But the age factor is also dependent on environment. It is quite possible for to live beyond the age of 60, with good living. As it is the case with Sargon of Akkad (or the Great), said to live to the age of 85. Rameses II of the 13th century BC, ruled Egypt for a long period, and died probably at 93 (I think).
In any case, back to case of Noah. The non-existent of this real Noah you are talking about, would not be possible to put age to him. All we have is the creation myth found in the Genesis that point his age to 950.
The only real Noah, is the original Noah, the Sumerian hero, Ziusudra, or his other variants (like Atrahasis or Utnapishtim) became a minor god, so his age is undetermined because of his immortality, but his father, supposedly Ubara-Tutu of Shruppuk, was said to have ruled for 18600 years, according to the Sumerian King List. So the real Noah (again, referring to Ziusudra) is just as exaggerated as the bible.