Can one place frozen challah on top of another pot on the blech on Shabbos?
Yes, this is fine. The challah is fully baked before Shabbos, and therefore even if the challah will harden a little, there is no issue of “cooking” it, and it can be heated on top of another pot.
See Shulchan Aruch 253:3, and Biur Halachah, who cites a dispute between Peri Megadim and Dagul Mervavah concerning placing a challah on top of the blech itself (the question being whether this can be considered mechzi kimvashel, being that bread is not baked on a blech). On top of a (full) pot it is certainly permitted.