Yeesh - that does sound very complicated! Long time ago, I played a game that kind of did something like that - Wizardry 7 was it? I never cleared it, though - because the other parties always grabbed the stuff before I did
I am regularly looking for offers of Wizardry 7 and 8, but even used, they are really expensive if not in bad condition. Unfortunately, I missed both of them when they were added to issues of some computer game journals for a good price some years ago. On the other hand, I guess I won't have time currently to play through these games anyway - I prefer to put the time into DC designs...
The NPC walls work really well - might be a good idea to use something like that, too. It sure makes interaction more interesting... I guess I can just turn my sprites into NPC walls (assuming NPCs always face forward, which may not be a good assumption).
I think it makes sense that the NPCs generally turn to the player, independent of their position (left/right/front), so most of my NPC walls use the same picture for all perspectives. However, I change minor details like eyes, mouth, hands, tails to add some movement effect. The interesting thing is to calculate the seize of the characters for each perspective...
For big characters or NPCs who cannot move, I draw different perspectives. Sometimes, I also use two walls - to simulate moving or in case the NPC shall appear right in the center of a square.
By the way, I draw the complete character in high resolution. I can thus use him/her for portraits/small pics, sprites and NPC walls.
But then encounters that do not use sprites will seem very uninteresting...
If the NPC is stationary, sprites would make less sense than NPC walls. To make conversations more lively, I switch between small pics of the talking characters and no pics (thus showing the NPC walls). Sometime, I throw in big pics - these are also good for hiding the party overview to the right if you want to imply that one of the party members is away for a minute. For important characters, I draw additional portraits (e.g. angry, happy).
Three scenes from the starting dialog of "Snow Tigress":
Here you have a small pig, a big pic (same base as the small pic), and NPC walls combined with a big sprite.
Of course, sprites are useful for moving characters. It also depends on your general philosophy using graphics. If you depicted only walls and doors, but e.g. no tables, chairs a.s.o., you would probably also not use NPC walls. If on the other hands you showed all kinds of furniture, plants, and other stationary objects, you should also show stationary NPCs in my opinion. The following animation shows a combination of sprites and NPC walls.
Note that the big NPC "Toratenno" is shown from different perspectives, while "Binjo" is always shown from the front as if turning to the player. The approaching and leaving helmettigers are sprites, and the last pic of the heavily armored tigers is a small pic based on the same drawing as the NPC wall.