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I have found a little more on this query. Mansion House was a legitimate place built in 1850 at San Jose, perhaps not fully finished when John Sampson was sent there?  It became known as Beaty's hotel by 1855 and there's a picture of it here
http://digitalcollections.sjlibrary.org/cdm/ref/collection/arbuckle/id/297

In the notes beneath it mentions "Steam boat line stage office for San Francisco." But it doesn't make sense if the intended destination might have been San Francisco. Maybe it all comes back to medical care.

The world state and the USA elections preoccupied me for some time, but I did find the arrival for the Pilgrim in a San Francisco paper. as follows
Shipping intelligence
Port of San francisco February 5 (1850)
Arrived
British barque Pilgrim Francis 77 Days from Auckland, with 54 passengers

Unfortunately that is all there was and there was nothing about the passengers except the number. Still looking for what became of Mary Georgina Scott. Not mentioned in the Probate for her Brother John Sampson, died san Jose 1850. All Mary's living sisters petitioned for a distribution from John's intestate estate but Mary Georgina was not one of them.  Her husband John was in Marysville Yuba, County California during this protracted probate, but later married as a widow, in England, 1854. So, it comes back to where and when she died. Between 1849 after leaving Pitcairn, arrival in San Francisco on 5 February 1850 and John Scott signing papers for John Sampson Probate in Marysville 26 September 1850.
My original estimate of arrival was based on the entry of Walter Broadie's diary about Pitcairn, and was written retrospectively, so incorrect for timeframe. So, the question still remains. At sea? In San Francisco, or somewhere between that town and Marysville.

Thanks Genealiza. The dates on those "Mansion" houses tends to confirm that San Jose is definitely the place he died.
The  pioneer files re the Sampson family were largely written by Alice McHone a grand daughter of Camilla Beverly Scott (Sampson) Klein and were written such a long time after the deaths of the 3 brothers, that they do have errors in them. Alice's grandmother died before Alice was born, so stories are not first hand. Therefore I'm cautious of that information and have where possible tested it against known family history and newspaper accounts, documents etc, from this side of the world.

Thanks for that Genealiza. I had tucked this away for future research. I do know where most of those children belong, and their married names.
It seems that the whole family were very familiar with litigation. 

Another clue, taken from the probate account,  might be in identifying where Mansion House was. I've looked in both San Jose and Marysville without anything conclusive.