I don't know if there is any connection with your Allen or not at this stage. My ancestor was Ann Allen born 23 May 1791, married my gggrandfather, John Sewell, on 15 Oct 1811, and died 10 Jan 1872 in York, Western Australia.
It is interesting that you mentioned a James Brown who married Mary Allen. My ggrandfather, Jame Brown b 1771 and died 1834, married Martha Edwards. I wonder if there is any tieup there.