This isn't really that massive in scope. I have an NPC for this outside midgar I use for testing purposes that works pretty much like this. (Of course the NPC is not in a public version, haha.)
Not sure what you mean with "It'll help out with Nightmare". Do you mean my mod or are you also making something nightmare titled?
giving rewards or disabling them is very, very easy like NFITC1 said
Make a dialogue option that sets or unsets "skip after-battle screen" in the fieldscript and shazaam, no need to even edit the monsters or anything
same can be done for no-game-over, no-victory-pose and so forth. There is no way, at least not that I've ever come across, to only one part of the battle reward.
It is technically possible to copy the entire inventory into variables, store them temporarily and then paste them again after battle (so whatever you steal / drop is pointless because it'll get overriden by the stored variables) but thats a LOT of variables ans a LOT of script. You'll need to run that as external script in mode 3 to make that work. Cloning the monsters is the much worse option because that will eat a gigantic ton of encounter slots.
as for the 'unlock' part, simply run a string of gamemoment checks, and for the weapons, a bit check on the weapon_killed variable (I have this somewhere in my PMs)
you'll need a bunch of copy pasted lists, each with one more entry and then have the NPC use a list depending on the game moment variable which then
What took me the longest, by far, was searching each field script for the gamemoment variable set after beating a boss in the story. The rest is almost just copy paste. You can even let the NPC set a custom battle BGM of your choice if you're feeling eager beaver.
I would feel bad for hijacking if I had feelings but on a topic like this, where clearly only a handful of fields in the game are touched and nothing else, is there a way to dynamically patch something like this into the field if the field isn't vanilla?