FF6Tools - Browser-Based ROM Editor

How many arguments do you have for command D3? I have 4 bytes total (including the D3). Here's my commented disassembly:

That is correct, but in your tool it uses only 3 bytes. 
For example event 74 (in front of Zokk's house, Map 32, trigger 130):

Code:

C86150: B1 02             Set Player Overworld Sprite to 2: Bartz
C86152: D3 80 E1 18       Set NPC 00 on Map Position $21,$18
C86156: D3 81 E1 18       Set NPC 01 on Map Position $21,$18

instead of address C86156, your next command is at C86155. I think you use the D8 parsing for D3

Do you have a ROM map for FF5 Advance by any chance?

Only the link @madsiur  shared. However I have accumulated a bunch of addresses in the Viewer that might not be listed. If you are interested I can copy them from the source code.

I guess the tedious way would be to make a binary search for partial or full SNES data in FF5A, what I did (and maybe you as well) for FF3us / FF6A.

Yes that is what I did for most parts
But don't expect 1:1 copy - GBA require data aligning so something that is 7 bytes in SNES is probably 8 bytes in GBA. Also in some of the data they have scrambled the position of fields. Also GBA has more weapons, classes, ... so some fields might get expanded to use more bytes.
Ofc, somethings are reworked from a scratch. Like scripts for example, use completely different system. Everything that is visualized - its stored differently (tiles, backgrounds,...)