Adrian
Having looked in detail now at all the records you posted, I tend to agree with you that Thomas and Ann DOWLAN never went to the Cape, and that perhaps Thomas DOBSON was Richard's stepson.
I have made one other discovery by going through records on FindMyPast. It was always said within the family thatCharlotte PICKSTOCK, the 'child' of Richard and Frances who sailed with them in 1820, was not Richard's daughter, and was born in Madeira.
Royal Military Asylum (Chelsea) admissions (British Army Schoolchildren 1803-1932)Taken from a transcript by Peter Goble
Charlotte PICKSTOCK, born 1809, aged 6, admitted 6 January 1815
Father: Isaac PICKSTOCK, Private, Royal Regiment of Veterans
Mother: Sarah
Discharged 5 January 1820
The RMA aimed to educate orphans of British servicemen in the regular army who were killed in the Napoleonic Wars. The 5th Royal Veterans Battalion was stationed in Madeira from July 1809 until September 1814, so Charlotte could well have been brought up in Madeira. Her discharge from the RMA in January 1820 fits with her sailing to the Cape with Richard and Frances in February.
Was Isaac perhaps Richard's brother?
Every breakthrough brings more questions