Can you before shabbos place a cooked chulent in a pan on top of an upside down pan
My understanding of your question is that you want to know if you are allowed to put an cholent that isn’t fully cooked on an upside down pan to serve as a blech, to allow the pot to stay there from before shabbos. Yes you may. The reason is because the upside down pan is now serving as your “blech”, and therefore even if the cholent was not fully cooked before shabbos, as the stove is now considered covered.
Shulchan Aruch O:CH 253-1.