"Instant Death" over-rides "Miss if prot. from ailments"

Your best bet is to write a custom Extra effect that takes everything you want into consideration, You should start with the "Cancel Enemy desparation attacks" code as a base (since I'm assuming you don't want the enemy to be unable to counter before dying) and copy it elsewhere, then modify the code to include everything you need. Then you add a pointer to the location of the code to a "blank" space in the pointer list starting at C2/3DCD, replace one of the lines with "8C 38" which is just a return, just not the one at C2/3DCD since thats the default that all spells with no extra effect have set. If you haven't made changes here before, you can put it at C2/3DD1 then set your extra effect to (0x02) in FF3usME.

Code:

Table for special effects code pointers 1 (abilities)

C2/3DCD: 8C 38
C2/3DCF: 8C 38
C2/3DD1: ?? ?? ($02) (Pointer to your code!)
C2/3DD3: 43 3D ($03)
C2/3DD5: F2 38 ($04)
C2/3DD7: 8C 38 ($05)
C2/3DD9: 8C 38
C2/3DDB: 8C 38