Re Donald Cameron, although the letter says he was from Southend it is unlikely that Donald himself was ever in Southend.
He was born at Barmollach, Kilmichael Glassary 24th June 1793, eldest son of Archibald Cameron and Ann/Agnes Cameron. Around 1810 Archibald and Ann moved from Barmollach to High Glenadale, Southend with the rest of their family. Donald stayed in Kilmichael Glassary and emigrated from Crinan on 23rd June 1819 after marrying his first wife. They arrived in Montreal on the 21st August. This information comes from a detailed memoir that Donald wrote later in his life.
His second wife was Elizabeth Armour, daughter of Alexander Armour and Jane Love. Alexander Armour was probably born in Trodigal, but farmed Belloch [Glencreggan] north of Glenbarr. He was in Kilwhipnach from 1814 for a time.
Donald Cameron bought Lot 30 on 5th Concession, Vaughan soon after his marriage to Elizabeth Armour in 1824.
Archibald Cameron, High Glenadale, is the ancestor of present day Southend Camerons.
Hope this adds a few more details to your fascinating letter.