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.
This is ALMOST true.
Add Wounded in front of imps and intellect devourers and then give them back their abilities - like sting and devour intellect and resistance.
OR...
Call them something different. Intellect Devourer could be Ustilagor, baby intellect devourers, and imps could be something like Imp Grunt or Fledgling Imp or Imp Peon. Just something to let us know they are inferior to normal versions.
But then they'd have to call phase spiders something different too, like Phase Teleport Spitter Spiders, and wood woads and mud mephits would need new names too, and harpies would be Stone Throwing No Club Harpies etc, etc, etc
