I have three sons prior to Elizabeth's birth, one of whom I believe to be John Henry. Three daughters also, post Elizabeth including Mary Ann.
I am unsure about John Henry because he was twice married. On his first marriage details it cites his father as Anthoney (in that spelling), on the second as Edward. Anthony would fit time wise and geographically its just the doubt cast by the second marriage.
I can find no further details about John Henry's birth or Anthony's marriage or birth. But drawing comparable time lines would suggest he could be the son of JW ( a stretch but possible), better George or Thomas (JW's sons) but I have no proof. Equally he could be from a parallel line of the family.
There was obvoiusly UK family in the early 1800's as George, his sister and John Augustus are recorded returning from England by boat. JA went to school at Charterhouse and died in Surrey. George died in Calcutta but his wife may have come to the UK. Madeline, JW's daughter, married Col John Geddes. At that time she was cited as coming from London not India.
There are still Hessing family members from the JH line living in Bombay.
Do you have any photographic records from Mary Ann's descendants?