Might the lack of records re his deceased father Johnstone and mother Janet be because they never came to Scotland with Edward and Thomas.
How common would Johnstone be as a forename? I have not seen it before, but it seems quite clearly written on the death record.
Johnston isn't actually an uncommon first or middle name in Ireland. Surnames were often used this way- sometimes the name was mother or grandmother's maiden name, other times might be from minister, doctor, schoolteacher, landlord, etc.
