Mechanics of Brain-eating
Many many years ago an issue of Dragon (I think it was issue 135; which ever it was, it had a witch and a scarecrow on the cover) had an article on illithids which I have always treasured called "The Sunset World." It had the following to offer on the subject of brain extraction, written from a more scientific viewpoint:
(The following is not a direct quote)
The illithid reputation for brain eating is actually a habit not of nutrition but of reproduction. Illithids are actually omnivores who eat a wide variety of vegetable and animal foods, although they do evince a culinary preference for both the raw and prepared brain.
An illithid's tentacles contain small detachable organisms, which attach to a creature's flesh when the tentacle strikes firmly. The illithid then telepathically directs this corpuscle to burrow through the skull (which must be done at extremely close range, hence the impression that the illithid must maintain hold on the creature) until it reaches the brain, whereupon it disrupts all higher brain function, leaving the victim effectively braindead. Once undisturbed, the illithid uses portions of its mouthparts like a bone saw to open the skull and inserts a tadpole-like larva, which wraps its tentacles around the brainstem and takes over the host's autonomic functions. The parent illithid then brings food for the host body and the larva lives off of its bloodstream as a parasite for several months, growing inside the braincase until it bursts the skull completely apart and emerges as a fully developed illitihid about two feet high; it will complete its growth later.
This is a nice, pseudoscientific take on the "ceremorphosis" idea presented in the Fiend Folio and elsewhere. The issue also contained a sister article on fauna of the illithid "homeworld."
Regarding the appearance of illithid tentacles around the mouth, I prefer the illustration from one of the Spelljammer Core books, the Concordance of Arcane Space I believe, which shows an illithid with its four tentacle splayed open to reveal a mouth of interlocking teeth; the tentacles are in a square arrangement, pointed out diagonally.