Ariadne Oliver very definitely meant to be AC, in some ways, anyway. AC reveals in her autobiography (I think) that she ate oranges by the bagful and she makes Ariadne do the same with apples.
Training maids - this wasn't a money-making venture. It meant Miss Marple employed untrained girls, probably straight from school, and paid them a pittance because while they were with her they learned what they were supposed to do. Then they moved on to a better job and she replaced them with another 14yo.
I assumed she lived on the income from an inheritance, either dividends and interest on inherited shares/other assets, or an annuity bought with the inherited capital, having inherited her parents' house. She never mentions having any brothers or sisters so maybe they all died before her. Her nephew becomes a successful author, so may have had a bit of money coming in to help his old Aunt Jane.
In the real world, someone like Miss Marple would have been in dire straits after WW2 because higher rates of taxation and inflation would have left her without enough to manage on. She'd probably have had to sell the cottage and move into some sort of residential home.
I can't remember now whether she still has a live in maid after WW2, or whether she has a daily woman/charlady. That would be realistic. In the books it's often phrased as difficulty finding any member of the uppity working classes willing to do domestic work, now they've all got ideas above their station about doing office or shop work, or earning a much better wage from factory work. But in reality the middle classes would have struggled to pay for live in domestic servants even if they could find anyone willing to do the work. Hence the au pair in A Murder is Announced.
AC started writing as a very young woman and lived for a very long time. In middle age and even more so when she was elderly she reflected ruefully that if she'd known how long she'd have been writing about Poirot and Miss Marple she'd have made them both a lot younger to start with. 