@ToraCarol: I told you to disable the line "InitRegion( 8, 0 )" from the "Main_Init" function because I thought you wanted to make a cutscene (not a playable sequence). Put that line out of the "if" block if you want the region to work even with Blank.
Note that you'll get back to have Zidane as soon as you leave that field. In order to keep Blank along the fields, you need to edit the following fields as well.
Making a cutscene/playable sequence is not easy at all in any case. Make sure you understand the changes I suggest and if you don't, ask why I suggested them. It may be a slow learning process but you'll eventually understand scripts and know what to do.
@TheHobgoblin: All your problems come from the fact that the Steam version is basically a port of the old PSX version to more modern machines. The source code of the PSX version has been turned to a C# code with only the strict minimal changes required (how they handle images, 3D models, menus...). The basis of the code's structures (and a lot of the game's raw data) is a copy of how things worked on the PSX version.
So, the things you want to do are possible but not with Hades Workshop. I think that Memoria's "enable all characters" option doesn't allow to have both Marcus and Eiko in the team at the same time. The list of characters you printed is accurate and that's what needs to be changed if you want no restriction.
Also, I warn you that editing the game's UI (the way the menus look) is something on which we had only partial results. I'm not sure of what could be the resulting display of the "Choose your party's characters" menu after all the changes.
You need to edit the game's source code. You may do it with the devs' version of Memoria but also with a generic tool that is called
dnSpy (if you work on dnSpy after doing the changes in HW, you can use both as I did for Alternate Fantasy).
In dnSpy, open the DLL of the game "Assembly-CSharp.dll". You'll see the list of classes (a structure of data) and their methods (an operation related to that class). If you know C# programing, it will be much easier (it's C# both for Memoria and dnSpy) but a few things can be done quite simply.
1) The total number of spells in the commands' sub-menus.This is can be modified in the class "rdata" (in dnSpy, that's in the folder "{} -").
You should already see a bit how command datas are initialized in that class. That's exactly the part of the source code that is modified by HW when you edit some commands or stats (except there you'll mostly deal with numbers).
The error "You cannot add spells to the list anymore" refers to the value of "_FF9FAbil_ComAbil". Find where it is defined and "right-click -> edit the method/class" to change it.
It works like this: "_FF9FAbil_ComAbil" is the list of all the spells that are inside a command menu (side note: the terminology between HW and the C# code can differ as I've started developping HW for the PSX version without any access to the source code ; the "spells" of HW are called "Abil" or "Active Abilities" in the C# code). On a non-modded file, it starts with Dagger's Summons (49 is Shiva, 51 is Ifrit...) and continues with Dagger's White Magics (1, 2, 3...).
Simply add a few entries at the end of the list with the spell IDs you want to give.
A bit above that, you have the initialization of "_FF9FAbil_ComData". It's all the commands in the same order as what is displayed in HW.
Dagger's White Magic, for instance, is initialized like this:
new rdata.FF9COMMAND(174, 458, 8301, 1, 16, 8UL),
The first two numbers can be ignored (it's a leftover of the text hexadecimal position from the PSX's version).
The third number, if I recall correctly, is a code for the help dialog's width and height. I think it is also ignored (the bubble size should be auto-computed on the fly).
The 4th number is the type of command. Here it is a command containing several spells (1).
The 5th number is the number of spells inside that command.
The last number is the starting point of the commands in the "_FF9FAbil_ComAbil" list. Here it starts at the 8th spells (the 1, 2, 3... from before).
If you want to use the spells that you added at the end of the list, put 192 as that last number.
2) Separating the slots of Eiko and Marcus (and Cinna/Quina and Blank/Amarant).I'll edit this message and write the way to do it later...
Most of the things involved should be in the class "ff9play". The chararcters have several kinds of IDs and the one adding the problem is the "slot_id" or "slot_no". One has to add more "PLAYER_INFO" in the method "ff9play::FF9Play_New" and change all the conversion methods "ff9play::FF9Play_GetCharID2"... There are surely a bunch of other things to do and they need to be done smartly if one doesn't want to change all the field scripts related to Cinna/Blank/Marcus as temporary player characters...
Note: The field scripts are not part of the "Assembly-CSharp.dll" datas but are inside the archive "p0data7.bin" so you can still add HW modifications on top of dnSpy modifications.
If you try to open the modded "Assembly-CSharp.dll" with HW, it will surely crash though, as "rdata" is a key class for HW and it is assumed to have a very specific pattern (plus the total number of spells is hard-coded in HW's source).
Sorry for the late answer.