Kind of replying as I read your post, so bear with me here.
For your playable DK Cecil and editing events: When your character sprite shows as a Mini it's most likely from the party member not currently being in the party. What I'd do here is:
1:Delete the "add Paladin Cecil to party" command since you aren't using him.
2:Delete the "Remove DK Cecil from party" command that happens shortly beofre the Paladin adding.
3:Delete the "Change sprite to Paladin Cecil" Command. If this is still there but you don't have Paladin Cecil in the party, it'll cause the Mini problem you're experiencing here.
4:Look through all the main story events after getting Paladin Cecil, and anytime you see "Change Sprite to Paladin Cecil" chcange it to change the sprite to DK Cecil. Should only take like 20 minutes or so to sift through em.
Also, a little tip: you can hit the / key when looking at any list in the editor to make the list numbered. This helps save a LOT of time, since you can jot the number of the event command you're messing with so you can find your spot again quickly when testing stuff out.
As for your other hack idea: working on your first idea first will definitely help you get a grasp of things! As for event editing, I'd say make a backup of your project and just fuck around with event commands a bit. Interesting ideas can pop up when you're intentionally trying to see what makes the game fall apart!
Using the Namingway's event for testing out event-related ideas helps a LOT as well, since you can usually find a naming way pretty quickly.
As for editing sprites, I'd grab YYCHR. since you can use ZNES savestates to see palettes easily, it REALLY helps take some of the stress out of graphics editing. I like to use 2 instances of the program at once: one with my project, and the other with some random rom I blanked a shitload of space out of. I'll copy/paste what I want to mess with to the blank rom, do my editing there, and copy/pate it back to my project when I'm ready to see it in action.
I'd also suggest taking a look at the Slick boards, as there's a section of the forum dedicated to FF4 Snes which has a SHIT TON of useful info. There's usually a few regulars that check the boards each day. I also have a graphics thread there where I share what I'm currently working on. There's a handful of patches of said edits throughout the thread (though I definitely need to edit the first post so I can link to them without people having to sift through pages of stuff).
Didn't mean to write a novel, but hopefully this helps you out a bit!