It's possible to do most of these things with and without 'hard editing' of the game's .exe, but a successful hex edit would presumably allow you to set the actual damage reduction of each thing and/or the behavior of certain commands and statuses.
-) Back Row: There's a formation setting that disables the 'Change' command, and presumably disables your Row too. It's used in the Pagoda for Yuffie's fights and in the Final Battle. No idea if it's stable or not.
-) Sadness: If this was removed from all enemy attacks and the Tranquilizer item then Sadness would be effectively 'dummied out' of the game and only available either by loading a pre-existing save game where a character had the status or through save-editing. It might be possible to add character pre-battle AI to remove the status to prevent this.
-) Barrier/MBarrier: I think editing to the .exe is the only way to alter these, maybe changing the damage reduction they provide or the speed with which they deteriorate. Wall and Big Guard could be removed to reduce their utility maybe but W-Magic and W-Item get around that. You could perhaps set these spells/Materia to not be compatible with All. It made a surprisingly big difference in the Hardcore mod when Big Guard only affected one person instead of everyone.
-) Dragon Force: The effect of this spell can be altered easily using Wall Market, either reducing the stat% gain it gives or changing the effect completely. Same thing goes with Hero Drinks.
-) Defend: Now I've never tried this, but I'm assuming you can change this in Wall Market again. You could set it to have the same effect as 'Attack' and change it's name, that way you have the option to attack even if the Limit Break bar pops up or maybe even if you have Slash-All, Mega-All, or Double Cut equipped (assuming they only affect the normal Attack command). Haven't tried actually doing any of this though.
-) Materia: A lot of Materia can have their levels reduced, which blocks off either the more potent effects (like Wall) or limits their effective use (like Final Attack).
Thing is, I wouldn't implement any of these for this mod (except Final Attack, I reduced it's max level to 2) because they're part of the mechanics and I didn't want to dictate to the player how they should play the game. You should be able to swing by without going full out with Sadness, etc. but it's there if you need it. This isn't a hard mod in the strictest sense, just something to have fun with. I'm planning to make a ridiculously unfair version of this mod in the future though, probably when I learn how to edit the .exe and the weather isn't so sunny and cheery.