"But then that means that human made devils on an archdevil tier have existed before"
That's how the original archdevils came to be lmao, but to the first question Devil!Mikhail is at the level of an archdevil if they dont use Almonde's core to further power themselves up. Ironically it's because he's fused to Lazarus that his power initially drops then flares exponentially once he entrusts it to the brat.