I'll just say that ideal RPG is one in which the player can make any decision that theoretically can come to his head. BG3 is great compared to other similar games, but there are moments where it could be done even better. Yes, even if he would have attacked me immediately after healing with the ability, they did not even think that someone would want to do it and see what would happen. I just wanted to see something other than a bug
Agreed. You should be able to heal the mindflayer. It is an NPC and you should be able to cast spells/give potions to any and all NPCs. After which, it should probably attack you. But maybe, just maybe (difficult persuasion check), it puts you into the category of "helpful servant" instead of "food." Or it takes a moment to search your brain, and recognizes that you have a tadpole in your head and thus it should probably try to keep you alive for breeding/Absolute purposes.
Then it could order you to do additional tasks to aid it (tasks that would obviously involve a lot of killing and kidnapping). If you disobeyed or questioned it, you'd have to pass a difficult charisma or intelligence check or it would become hostile. Now that could be an interesting playthrough: having to follow the mindflayer's instructions or risk having your brain eaten, always a single failed deception/persuasion check away from it realizing that you have too much free will and it would be better served eating you.
I could see you going to the grove offering your aid to the tieflings, except instead of leading them to Baldur's Gate you lead them directly to the mindflayer >:)
Or the mindflayer tells you to go directly to the goblin hideout to coordinate with the other True Souls...