First Problem- A generic PC would fix this, and right now is the only fix because SPC battle animations are incompatible with other characters by every means I've tried (so far). You could just add it to the 64 optional skills list (the one you use when you add spells with scrolls). Heck, if you want, I could just make an item code to force, say, Halcyon Scroll to teach Lapu Do. I don't think I've released the item property codes yet, but I do have them.
using Hiiragi for the sake of Lapu Do
However, if you don't care how you get Lapu Do as long as it's not in the 64 optional skills list, you CAN alter what each class can use and at what weapon level, *OR* you can just add Macabre Dancer as one of your classes (the Job title is nothing but text, so don't worry about it). I don't remember what codes I've made available, so you may need to request these.
Second problem- I'm not sure what the problem with the boss is (unless you're talking about storyline bosses, and even then, it'd be strange), but you cannot pick up key items with non-Haseo characters as they don't have Key Item listings. Furthermore, if the game gives you access to the Key Item menu (usually happens if you switch back to your Item/Equipment menu from another character's), do NOT go into it or the game will freeze.
Also, while Reminisce granted other PC's bike access and telekinetic hits (read: the ability to hit without moving), all volumes will force the kick animation for trap boxes, and no character has one save Haseo (also save Taihaku, whose AIDA shield animation is called the same way, but it doesn't hit the chest so...). Since the characters can't kick, you just run after the chest forever. Use the bike.
Oh, and exiting the Arena either forces your character to be Haseo (when you lose) or freezes (win). That particular part of the code is one of only a few things that assume Haseo is the leader and no one else. Unless you can find the assembly code driving the Arena exit event, you're just gonna have to live with it.
Granted, I haven't tried this since finding that the 2nd form Haseo is supposed to fix some bugs (such as allowing you to change equipment) so feel free to just facepalm at me if I already found the fix and don't know due to not testing.
Here's a summary of form change effects:
1st Form: First class only, normal equipment menu.
2nd Form: First two classes only, normal equipment menu.
3rd Form: First three classes only, allows the party leader an extended menu should Haseo have it. Allows Multi-Trigger.
Xth Form: Allows access to all four classes, and allows the party leader an extended menu should Haseo have it. Allows Multi-Trigger. Allows Haseo Double Trigger should he have this form.
NOTE: Haseo also gets a special menu as a party member in 3rd or higher form that allows you to select his current weapon slot; trying to use that slot directly will result in a freeze.
Also note that the Multi-Trigger doesn't reference Skill Trigger weapon changing per se (since only Haseo can do that currently); it's actually in reference to showing all known skills in the menus instead of just those of the currently used class. This is why Sophora can use her healing spells when using dual swords (3rd form) and why Matsu's broadsword skills are hidden (2nd form).
As for the extended menu, you pretty much have to disable it by downgrading Haseo if you need to change equipment since no one else can use it. Like the Key Items menu, it's proprietary to Haseo.
Third problem-You should expect such bugs every now and then, but yeah, it should be possible
to some extent. I have not tried a full game run, and I don't think anyone else has either, so do take some notes for us.
It's just that I've been a fan of magic since I was playing Final Fantasy 2 on the SNES (4 by official numbering) at the age of 4 (I learned to read when I was 4, yes) and was really hoping to play through as a mage-rogue.
Which is why I said what I said in my custom Adept Rogue video's comments section. If that video were the first I saw of G.U. and then I learned the truth, I'd be really pissed.
If it makes you feel any better, Haseo can use Magic on the Skill Trigger in Rebirth (with hacking anyway). I mean, it really sucks after a certain point due to the level ignorance bug, but still, it's there. You could throw Status spells on it instead (healing spells don't work). :\
Fourth problem - Grein. Sakaki's normal form can be fixed as well if you'd like to use him (I thought I posted the codes somewhere in this thread...?).
Now as for Sakaki <Zenith>, Gabi, and Matsu... well, let's start with Gabi, Ovan, and Matsu. Rebirth used a different format for accessing Skill animations in Rebirth, which is made clear just looking at the game in Hackstract; you won't find the Skill files in there. In Reminisce and Redemption, the access method changed, and Gabi, Ovan, and Matsu weren't updated. Hackstract won't show listings for their animation files (Matsu as it concerns his sword), but it will for everyone else. IOW, the original Skills they had (Tiger Blitz thru Bone Crusher, or Thorn Shot thru Plectra Bullet for Ovan)
may still be there, but they're inaccessible.
Sakaki <Zenith> is an entirely different case; that model has no Skills animations period. In fact, its model file listing simply lists the main model twice instead of main model and Skills animations. Even if you fixed Gabi, et al, <Zenith> would have to be a generic PC to use Edge Punisher Arts.
Now that's not to say Gabi, Ovan, and Matsu's sword can't be used with the original character files at all. If you put aside Arts, those characters
can be repaired with our current knowledge and be made usable. Being limited to three attacks, Items, and no Skill Trigger (unless you swap a mage's model with one of theirs to use magic) might suck, though.
Oh! The codes to fix Haseo's 2nd form (for the most part anyway; I might be able to fix him almost completely if I overwrite certain animations for others) are definitely in this thread, so that's another Punisher you can use in Redemption.
The Commentator is a generic PC, which Ranylyn isn't going to use just yet. He is also an event PC for reasons unknown to me (just like how IYOTEN and Asta have generic PC variants for seemingly no reason, as they're never used).