From No 8 ......'her occupation (I just worked out) was “plain cook” - meaning she could cook most meals by herself or with the employer. No fancy sauces, and no housework!'
A plain cook, at least in NZ, does not really have the meaning you have for it.
A plain cook was skilled at being able to cook a range of food for all family meals so casseroles, roasts, puddings, jam making, baking for a range of numbers of people, budgetting/ordering/quantities etc.
My grandmother was what was described as a 'plain cook'. She therefore had the expectation on her to board and provide the meals, at least for visitors to the isolated valley where her family farmed. This included various religious visiting to minister to their flocks, stock buyers etc. When she moved to town she boarded new residents to the location such as young single workers and again religious until they were settled. At one stage when she was quite elderly she relieved for a very short time, when the owner of an 18 single men boarding house died suddenly.
I think the concept of what plain cooks did may have varied in the colonies like Aus/NZ from England. Plain cooks are still valued in farming families where the farming couple are expected to provide the meals for single shepherd/s, shearers and fencers and during lambing. My bro in law's family, when his mother was elderly would hire in a plain cook to cook especially during lambing when they seemed, to me, to have huge roast meals 3 or 4 times a day, after a lambing beat. The key need was someone adaptable as far as numbers and types of food.