] And there was at least one instance where the eldest son acted as a mediator between his mother and Shackleton. I had the impression that the boys were adults then, and not in the house? Anyone remember this better than I do?

... of Elizabeth Shackleton's sons being in the army? This may explain their absence. Her eldest son mediated between his mother and her violent husband indicates they cared for her to an extent(running blind here!)Perhaps when the sons were away,they relied too much on their uncle to protect their mother when Shackleton grew excessively violent. The fact not even she mentioned her odious domestic treatment in her letters indicates to me,as in our era,sadly the domestic situation was 'hushed up' to prevent gossip and protect the name of the family; Hardly a help to the woman.