I think people often oversimplify Christie’s view of the world, and your article helps set the record straight. Far from championing a rigid, authoritarian view of the world, her novels subvert the readers’ sense of stability and require them to look beneath the deceptive external trappings.
Her novel The ABC Murders could be read as an allegory of how the wealthy ensnare the working class for their own mercenary ends. In fact, the poor and underprivileged are seldom if ever the villains in a Christie novel.
I also agree with your assessment of Raymond Chandler’s conservatism, but I’d recommend the later hard-boiled novelist Ross McDonald as an alternative.