I think confusions are developing here from understanding of occupations. A person might be an heir of a property or acquire a property and use it for their name, but still be a merchant. Also a younger son of the main house has a choice of taking up a trade or working for his brother on the property. Merchant in Moffat doesn't preclude having a property a few miles away. On the other hand a Writer in Edinburgh might be a legally trained son of an estate, or he might acquire the estate through acquisitions in loans or bankruptcy of clients, or simply purchased from acquired wealth in legal dealings. Johnstone was a substantial family in this area, both titled landowner and peasant farmer. I cannot find an immediate reference to Johnson and Selcoth , but I'd stick to Moffat connections. The writer in Edinburgh could well have absolutely no connection at all. Good luck
