etwan = at any time, in any case. (Here is my secret source of wisdom, in case anybody wonders.)

unbeständiger als = less constant than

Summary of the poem: Evil women are so evil that their evilness could not be fully described even if all the stars would write to a paper as large as the sky with as much ink as there is water on earth. "May the devil in hell describe it!"

Max Reger's lied op. 75 No. 5 "Böses Weib" (lyrics slightly shortened) nicely captures the baroque humour. Not currently on YouTube, but the dots are at IMSLP – the piano part is particularly evil; der Teufel in der Höll' spiel's!