As to the mistress part, I think the historical consensus is "can't be proven one way or the other". August had up to 200 known illegitimate kids, and she was definitely a favourite, but "having sex with anything that moves" doesn't necessarily translate into "being into incest as well". It certainly was gossip at the time, hence Wilhelmine mentioning it casually. At any rate, she was an interesting character; wiki sums up why here. (An entry which also illustrates why a gay boy still uncertain about his sexuality might have been drawn to her. He retained good memories and wrote to Voltaire decades later: "In my tenderest youth, she inspired two passions in me - you may have guessed, it was love and poetry. This little Miracle of nature, equipped with all the charms, had taste and tendresse and tried to share both with me. I succeeded with the lovemaking, but not so much with the poetry." (Disclaimer: it's always possible he was het-posing for Voltaire, of course, but given how insulting he could be about most people, especially women, this is still a remarkable Quote imo.)

The six: over to Mildred of Midgard.