What is your source for Sarah being born in 1800 in Danville?If it is a better source than the 1850 Census, I will accept it.But even if Sarah was born in 1800, I still submit she cannot possibly be the daughter of Charles & Jane SIAS.
You seem to be saying that because there would be only ten years between the birth of Samuel (in 1790) and the birth of Sarah (in 1800) that this is a trivial gap in time, but it is not.It is an insurmountable gulf.
Jane is said to be born in 1743, which date I've found no reason to doubt.Even granting that Samuel was born in 1790, she was then at the limit of her fecundity because she was 47 in 1790.There is no way she had a child in 1800 (at 57), much less in 1805/6 (in her 60s).Her biological clock had run out well before 1800.
Your suggestion that Sarah might be "a daughter of one of their children (James or someone else)" is exactly what I had suggested.On that we agree.
As for raising the issue of adoption or illegitimacy, that possibility can be raised in regard to anyone's birth -- "It's a wise man who knows his father," and many an adopted child never finds out they were adopted.But lacking evidence to support such an allegation (e.g., court records), it's not a fruitful way to explain anomalies in a pedigree because the vast majority of anomalies in pedigrees are the result of making a wrong connection, which appears to be what has happened in this case.
The locations of "Derby," "Newport," and "Coventry," as given in the 1850 census, apparently refer to the townships, not the towns; so, yes, they do abut one another.The fact remains that George & Sarah (SIAS) SMITH were living in same township as James & Patience (DWIER) SIAS through no less than three censuses, and that cannot be said in relationship to any other SIAS couple.
I don't believe the parentage of Sarah has been proven to be James & Patience -- the evidence is entirely supportive, but circumstantial.On the other hand, I believe the evidence clearly shows it's impossible for her parents to have been Charles & Jane SIAS.And when the scenario doesn't fit the data, I believe in revising the scenario, not dismissing the data.