RE: "If the completion of an adventure causes a new monster to appear, you draw the replacement adventure before placing the monster."
I would argue that the monster appears as part of the reward for the card and therefore is acted on immediately before the card is replaced. This is my interpretation.
The FAQ covers this
Q: Can monsters that are part of a ‘reward’ be placed on the new adventure entering play?
A: Yes. When placing monsters, replace any missing Adventure cards first, and then place the monster markers.