Responding to OP:
- The fear of losing oneself or changing to mindflayer is always there, all the way to the final battle. (This is toned down a lot compare with early access. It become a none stop temptation. Some like it, some hate it.)
- Characters using illithid power or not, are not changed due to Orpheus power, which he inherited from his mother. We only know this at the beginning of act 3.
- Shielding is not equal to solving. Tadpole still in the head.
- Doesn't matter which path, good or evil, characters are forced to face the Netherbrain because of the above. Walk away from the Baldur's Gate city is not an option.(Basically the todpole is the plot device to force characters to fight the final battle).
- Characters safety (or any consequence using illithid power) is only certain after winning the final battle.
All tadpols are destroyed with the Netherbrain. And we only know this option after we win. Before that it was just a "wishful thinking".
Orpheus, or his mother's (Gith), power is the only way to keep one from losing to self. That power allow Gith to rebel and almost wipe out the ancient illithid empire. It was mentioned a few times in the game. (It is official 'lore' of the D&D world.) So it is not as simple as "strong mind".
So yes, it seems silly to not use illithid power, if and only if, we look back at the whole BG3 event. However, to the characters, they don't have a crystal ball gazing into future beforehand.