I think that at least some of the case for fighting those high-CR monsters at the start could be fixed simply by adding the words "injured" or "badly wounded" (or something to that effect) in front of the name, so that you're not fighting a full strength Intellect Devourer, you're fighting a "Badly Wounded Intellect Devourer". Larian wouldn't have to change anything else about the enemies, but the qualifiers would serve as an adequate hand-wave for being able to survive those fights.
