I think this question of growing the higher bodies can be more accurately explained in terms of the Cassiopaeans' concept of receivership capacity. I think cross referencing what the 4th way stuff says about creating a real I with what the Cassiopaeans say about higher densities and how they interact with us can provide something of an answer.
Let's review the default state of a person not involved in esoteric work. They consist of many i's. There is one i that is very selfish and consumptive and likes to go to the mall on Saturdays and spend money frivolously. There is another i that likes to go out to charity events and volunteer at soup kitchens for the homeless. These two i's are antipodes and their efforts and tastes basically cancel each other out so that the overall organism/collective of i's merely spins around in a circle without being united by a common aim. In the average "John Doe" these days, the fragmentation is severe enough that when his contradictory behavior is pointed out to him, he merely rationalizes it as "preference" or "moods," and sees nothing particularly the matter with it. It is a logical deduction to say that these two i's are completely unknown to each other, and they are even unknown to the incarnated consciousness, if there is one, until it learns to self-observe. These i's are separated by a rift formed from a low level of being and lack of knowledge and awareness. According to Gurdjieff, this type of person is "dead inside" and lives life completely under the slumber of A influences, so that when he dies, nothing is preserved because nothing was ever solidified/crystalized in him in the first place.
The object of the 4th way techniques such as self-remembering, conscious labor, non-identification, etc is to build a permanent I that is no longer subordinate to the winds of fate, but in control of its own destiny. As the I is perfected it moves through different levels of refinement such as man number 5, 6, and 7. Theoretically, a man number 7 would have learned all of the karmic lessons of this density and would be ready to move on to the next one. In a higher density, he gets a more rarified body with greater capabilities that is subject to lesser number of laws. He moves into a higher cosmos that has commensurately fewer laws. Perhaps this could be the "mental body."
But this man number 7 is only one I. There are different versions of himself in different timelines and different universes. Some never had any interest in esoteric work, some are very far advanced, some are STS and some are STO. In some timelines he didn't incarnate at all. Some multitudes of these I's eventually did get to 4D, but they are all more or less on their own separate paths with their own agendas. Let's look at this in the context of this quote.
session941210 said:
Q: (T) Okay, you are riding on the crest of this wave in 6th density, is this true?
A: Yes. We are you in 6th density.
Q: (T) You are we, that is me T***, Laura, J*** and F***?
A: Yes.
Q: (L) Are you alternate selves extending into higher densities?
A: At your current reference point in space time, we are you in the future.
Q: (T) We are your destiny?
A: And vice versa.
Q: (L) You are not, by any chance, one of those weird ant or preying mantis beings are you?
A: Yes and no.
Q: (T) You are just another part of ourselves? You, us, the Lizards, the ants, the grays, the trees...
A: We are your whole self as you/we are in 6th density.
Q: (T) So, what we are working to become is You? You are us?
A: Yes.
Q: (T) So, when we move to 4th density and become whole with ourselves, we will know you also for a short time?
A: Not whole yet when at 4th density.
Q: (T) But in order to move to 4th density...
A: Closer.
Q: (T) We, us, in this room, are closer than others are?
A: No. Closer when at 4th density.
Q: (L) When S*** was under hypnosis she described seeing a fleet of space ships "riding a wave" and this unnerved her. She felt this wave was a fearful, invasion-type thing. Was this you and your wave she was perceiving?
A: Wave is transport mode.
Q: (L) Is that transport mode for many beings?
A: Yes.
Q: (L) Are you coming to invade us?
A: No, merge.
So one is only "whole" at 6D, where all of the I's in the lower densities, STS and STO, are integrated into the
I of the oversoul in a realm of perfect balance. This would correspond to the "causal body," the only body in the 4th way literature which is considered immortal. All lower bodies are subject to the cycle of reincarnation and are impermanent. From the point of view of 6D everything happens simultaneously, but as a 3D fragment stuck in linear time, these higher bodies appear to be grown in our future, because we are currently working to create a vessel of Being which can receive enough higher impressions to reconnect with the whole. This is why they say "we are you in the future,
at your current reference point in space time," when it is the present for them. This is similar to the fragmentation of the little i's in the crucible which are fused into the I of the individuality during esoteric work in 3D, only the scale is different. We are sort of like the little i's of our 6D selves. I guess this is why they say helping us helps them, because the more we work on ourselves and integrate ourselves, the better they are able to work on themselves and integrate themselves, making it easier to merge at some distant day in the "future."
It is a somewhat theoretical understanding of the bodies, but an explanation which I think jives very well with the observations from what has been ongoing with the Cassiopaean experiment.