As an RPG Maker 2003 gamedev I can guess the lack of run button and diagonal movement is due entirely to the caterpillar system (what allows your party members to follow behind you) so you’re entirely spot on with engine limitations! It’s technically not possible to hard code that system into RPG maker 2003 so it must be done through a series of on-map events (so four NPCs if you will). If handish used kentona’s Cooler Caterpillar System like I did for one RPG Maker 2003 project it’s possible it would just be way more trouble than it’s worth to edit that code to react to both a run button and a diagonal movement code (which tend to be buggy at best in my experience in 2003, the thing’s quite finicky). Though without the caterpillar system a run button is 100% possible, though the standard speed in RM2k3 is generally fast enough to not need one, CogDis simply uses a lower than average movement speed for the engine. I imagine this was done to emulate EarthBound’s standard movement speed. (As a side note, the movement speeds don’t really have nice middle-grounds with each other. Standard is good but a bit fast and one note below it is probably what CogDis runs at and is a bit sluggish at times).

I’ll definitely agree that designing areas around diagonal movement despite the lack thereof is a bit odd but it’s not the first rm2k3 game I’ve seen do that.

The difficulty definitely sounds like a lot though. I remember trying the moon dungeon in the last build of the game (which I’m guessing leads to the best ending in that chain?) and I just gave up after getting brutally wrecked lmao.