I managed to decode the game genie code to find how the game creates random battles, turns out it's as you described. Thanks!
I am also interested in modifying which monsters appear in which maps. For example, outside of the first castle you can fight slimes (00) or crows (01), but the game also seems to in some way store the quantity of those monsters which can appear in each monster group and also how many monster groups can appear. I have no idea how the game might store this information. Maybe it works similarly to other DQ games?
Thanks again, and if anyone can shed light on this issue I would appreciate it!