Putting aside the issue of visual variety and just considering what's optimal, II + SI:Div needs two spells to penetrate, ST needs only one.
True, although if you have a thief with detect illusions, the former can be removed without using any spells at all. So it isn't completely clear cut.
Technically the character needs to be a Thief+Mage to do that. Don't they ? And a good thief at that, or bad, but one that's driven that way. So Nalia is likely not going to do at that so great.
Besides, it's just one defense wall... they still got their AC, magic+elemental resistances, hit points, etc to get through that might as well be easier to over come with other ways. The ability to hit the opponent is usually not the only requirement to be able to kill them... or there would just be Conjurers all over.
Welcome to the sanity, you are free to search for the limit, it's out there, we drew it in the sand.
Here's how to install all the stable mods you ever really could want to Infinity Engine games.