yeah, i don't think that part is necessary

what is necessary is sound isolation, and those stone, sometimes single piece solid stone, sarcophagi are very sound insulating, possibly they did at one time have actual soft, fluffy, acoustically absorbent material like any form of cellulose on the seals, and yes, probably they did not have any ventilation, every hole for air is either a way for sound or a source of sound in itself, as well as air movement causing hair movement causing sensation

it is in fact perfectly safe to be in a mostly airtight container for at least several hours, and it is not impossible for the user to shift the lid and get air, at least, in most of the designs i've seen

the key thing is that most of them are made out of material that is extremely acoustically reflective, like granite or similar, and if you place a box like this, part way underground, in a space segregated away from people, with multiple buffering areas with lots of acoustically absorbent materials, and walls tilted to ensure that the only sounds that come down could maybe come from the sky

lots of details but i really didn't like the water part of sensory deprivation tanks, and i've been in them on three occasions, and i think it detracts from the experience actually

and i'd probably argue that a somewhat elevated level of CO2 probably does no harm to reaching theta either

another thing that helps us get to sleep is also cool, so there is that, if there is a gradient where you have a hot bath first, and then sit in one of these boxes, that would assist in stimulating the slide down to that state

hypnagogic state is a lot related to this, i can say that for a very large part of my life, until maybe the last 10 years, i have tended to fall into this state readily on a regular basis, often during daytime, and what you say about theta helping with learning, it would explain a lot about my above normal learning ability