Some really fun stuff here. I'd never heard of or thought of the theory that Sailor Cosmos is an embodiment of ALL Senshi! I think I like that. I at least want to ponder it more.
As for head cannons of my own...well I have a minor one. I'm currently rewatching the Viz dub of Sailor Moon S and I was, again, wondering why Kaolinite was given so much screen time rather than having the Witches 5 from the beginning or near the beginning, like in the manga. What occurred to me the other day was, maybe the Witches 5 weren't even done yet or at least are very new creations. Crystal shows the Witches being grown in a giant test tube and that's a cool thought. So, in my mind, the Professor allows Germatiod to possess him. He begins his Daemon research and has a few limited successes (such as that thing in Haruka's garage made from a Mugen student). He finally perfects the conversion of his human assistant Kaolinite (sorry if I'm misspelling names lol). I imagine she didn't resist or even volunteered out of love for him and lust for power. Intrigued by her conversion and the creation of perfect Daemon Eggs (hence we now kick off the season by attacking Rei for her Heart Crystal), the Professor experiments with creating human/daemon hybrids from scratch sort of like Frankenstein (Professor Tomoe strikes me as the kind to always be experimenting and pushing the boundaries of science for the heck of it). While Kaolinite searches for the Talismans, he tinkers on the Witches 5 (using different techniques and processes hence creating their vastly different powers) and completes them around the time of Kaolinite's death. Fortunately, this means that he has someone to now turn to for seeking Talismans (his choice of Eudial was probably arbitrary and primarily because she was who he was talking to at the moment).
That went into a lot more detail than I intended but it was fun to write and gives a theoretical behind the scenes look at the Death Busters. I like thinking through these kinds of details