Damn. Not even a sucky translation can stop that scene from being epicness personified. Except...the transformation. I dunno why, but the EG form doesn't have quite the same "OH SH**" quotient as watching evil ghost Zeromus turn into freakin' cosmic horror Zeromus just as the final boss theme picks up.
The only sane reasoning for the image change is that FF4 came first and someone thought it was a good idea to make the game a bit more district... besides changing his battle script.
I always figured it was just an alternate design they came up with during development that was rejected for the original but put to use in Easy Type. That's even more likely if they got around to actually making the sprite before rejecting the design.
And yeah, that all-party Doom number...that's cheapness personified, and this is coming from the man who magically goes back to full HP in the middle of the fight without any visual cue whatsoever.
The multi-target Doom/Count only comes when it's near death, so you should be able to finish it off before it runs out with little trouble. It does have a single target Doom that it can use throughout the battle but I rarely see it, I'm not sure what exactly triggers it.
The Call Books is the FF4A's terminology of the regular Grimoires. A random summon (other than Asura) is selected from all the standard summons (minus the special monster-item dropped summons) with a spell multiplier of 8.
The item drops are radically different in FF4ET. The Call Books tend to drop relatively frequently (Imps/Goblins are the best bet), which already existed in the original game as rare drops. These items are not really that useful once you are able to face Asura or Leviathan, but allow you to breeze through the original game quickly if you wanted to...
Yeah, I named them "CallBook" in this hack because I felt it fit in better with the rest of FF2us's item names.