Sometimes you have to chuck it all and start fresh to get to the bottom of things. It occured to me that the M may not be entirely irrelevant as a symbol, but I approach this new theory with cautious optimism.
First, if the Lesser Arcana are to be seen as representative of the Lesser Hallows, it has to be understood how Waite viewed the Lesser Hallows. From
The Hidden Church of the Holy Graal:
“The Hallows of the Graal legend are the beginning of its wonders and of its meanings only; but, as I have intimated already, the greater includes the lesser, and that which is of all the highest has assumed from the beginning in its symbolism the things by which it is surrounded. As it is in the light of man’s higher part that we are able to interpret the lower, as the body is explained by the soul, so even the Castle of the Graal and the great Temple, with all their allusions and all their sacred things, are resolved into the mystery of the Cup, because there is a cloud of witnesses but one true voice which is the spokesman of all. There is obviously no need in this place—as we are concerned with the greater subjects—to lay stress upon the subsidiary Hallows as if they were an integral portion of the Holy Graal regarded symbolically. They are of the accidents only, and as such they are not vital."
By “accidents” Waite refers to something external only and nothing of deeper symbolic significance (see #14 at this
Wiktionary Page). The Cup holds all the symbolic signifiance and the other Hallows can be understood by an understanding of it.
So what is the significance of the Cup? In the Ace of Cups it represents the Eucahrist by all outward appearances; but the bread and wine are only veils, the truth lays hidden behind them. Behind the veil means the world within, that of the soul; so that which is behind the veil is that which is within. And what is it within which the bread and wine symbolize?
In 1891, Waite attempted to form the Order of the Spiritual Temple and had a prospectus drawn up which is included in R.A. Gilbert's
Magician of Many Parts:
“There are also four chief processes in Mysticism – Regeneration, Illumination, Dedication, and the Mystic Marriage, or communication with Deity. These will be represented in the four divisions of the service – Regeneration through Aspiration by an opening aspirational rite, Illumination by the instruction of lessons and discourses, Dedication by a sacrificial service, the Mystic Marriage by a Eucharistic rite."
It can be seen from this that as early as 1891 Waite viewed the inner significance of the Eucharist as a Mystic Marriage (communication with Deity), that is, the reuniting of soul with the Divine within (the Christ). On the Ace this is symbolized by the marriage of the wafer (spirit) with the cup (soul). But these are veils of the true marriage within.
Also, from Waite's Fellowship of the rosy Cross "Return in Light" ritual, 1924:
"The sacrament of the Eucharist is the sacrament of that reception, the significance of which is that the Spirit dwells within. And this is the Christ Spirit. The secret tradition hereof is a great memorial concerning a great experience. The macrocosm is the body of Christ, the microcosmic soul of man is in the image and likeness hereof; there is union between Christ in the macrocosm and Christ in the soul of Man."
Everything about the imagery is a veil hiding an inner truth, and one has to suppose this is true of the letters as well, the "W" being a veil concealing the M. It may symbolize the Mystic Marriage or just Marriage; but I’m not convinced of this and will have to withhold an opinion for now, but it does seem to fit. If one assumes M stands for Mystery, the "Mystery of Christ" would here seem like a good candidate, probably better than Marriage actually.
So how do the other Hallows fit into this picture? It's as Waite said in the previous quote:
"As it is in the light of man’s higher part that we are able to interpret the lower, as the body is explained by the soul, so even the Castle of the Graal and the great Temple, with all their allusions and all their sacred things, are resolved into the mystery of the Cup. . ."
He uses the metaphor of a body, the higher part of which is the Cup. That which lies behind the Lesser Arcana generally is that which, more specifically, lies behind the Cup.