hi Desecrated
i'll try to tell a bit from my studies in chassidian kabbalah (and magic).
don't know how much use it will be to you, but perhaps some things will give you a new lead or inspiration where to look further...
i'm still quite a newbie in the world of magic and kabbalah (close to two years of, intense, studying), so that's certainly nowhere near real knowledge about these crazy big wide deep topics...
i never read about waite or his writings about tarot before (which book would you recommend?). i just know the deck quite well...
...having said all this in advance, here my stab at answering your questions:
what does the TAU cross mean in golden dawn, in freemasonry and in christianity? no modern meanings. does it tie wiht the birth and rebirth theme in the magician?
i’m not sure if i understood you right here – i suppose you mean the letter „tav“, the last in the hebrew alphabet? i know that this letter has been written as a cross in earlier times, and that in english (which is not my mother tongue) „tav“ is sometimes writtes as „tau“...
if so, this letter means the full length of time in our reality. tav carries the value 400, and the value 500 in the bible always points towards something which will take place in another reality, not the one here.
so, if tav is ment here it could mean that the hanged man realizes how he is trapped in the 400, in the ongoing reality of this time, which never stops. in order to get out of it, one has to do something completely different – like jump out of time, out of the universe (or stand on your head)...
in this sense it does tie with the birth/rebirth theme, because you have to „die“ in this time here, to get born into the other reality...
why did he change it from the left foot to the rigth foot?
maybe he wants to stress the situation of the hanged man as a person who ist so rooted in his spiritual being (his right pillar, the male side) that the outside world (which in chassidian kabbalah ist always represented by everything female) to him looks like uspide down, inside out...
this makes actually more sense than being hung up on your left pillar-side, which is symbolising the world of matter, the female side (i'm not 100% positive this is correct - if anyone knows better, please tell me!). that would be our normal state, hung up in the world of matter...
were there earlier tarot-sets all with the hanged man hung up on his left foot?
upside down triangle?
water, femininity, or just trinity?
water in the chassidian kabbalah stands for „time“.
water in hebrew means „majim“.
the letter m(ajim) carries the value 40, which places it as to be the little sister of tav, 400. while m(ajim), water represents the state of „being in time“ (think of jonah in the belly of the whale, in the deepest waters), tav represents the whole amount of time, the endless flow of time...
it has a connection with femininity, insofar as everything (!) that pops in existence, into matter, is considered female in chassidian kabbalah (and everything) male points towards the hidden, spiritual world (that's the way they read the bible, friedrich weinreb at least).
if waite ment to show this, i don’t know.
„it has been called falsely a card of martyrdom“
so why the gloria?
illumination?
whom are you citing here?
the hanged man is numbered the twelft card of the great arcana. but he is the thirteenth card in the entire row since the fool is the first.
and the number thirteen in christian/jewish mysticism is, as far as i know, closely related with jesus/the messiah. he is the hidden thirteenth; in chassidian kabbalah there is the picture of the rose with twelve blossom-leaves, six red ones and six white ones (the red symbolising the north/the bodily realm, the white standing for the south/the realm of spirit) – and there is said to be a thirteenth leaf, which has a colour that cannot be described; because it isn’t describable in this reality.
the glory which lies in the martyrdom is key to christian/jewish mysticism, as far as i know by now – more about that, see below...
„it expresses the relation between the divine and the universe“

i thought that was the same.
in chassidian kabbalah (sorry to repeat myself), there is always this distinction between the world in „flowing time“ (the universe) and the world of eternity (the divine), where there is no time.
so, perhaps, this phrase (whom is it from?) wants to point at this, the position of incarnated spirit who is trapped (hanged man) in ageing flesh and still feels the truth of there being a reality without time (from where all the universe-in-time flows)
the messiah (the hanged man?) is said to stem from TWO houses: from the house of david, and from the house of joseph.
this means that he is the son of david the great king, songwriter and vanquisher of goliath, AS WELL AS (think of the symbol of the cross, escpecially the rose in the middle) the son of joseph, whose name means „there will come another one“.
david, „the loved one“ stands for the side of eternity, of spirit, and joseph for the side of us which is "here" always knowing that this "here" isn’t all there is...
and the messiah is the son of both of them.
those two sides have to be reconciled in each of us, which is also the meaning of the cross in christianity, and i guess in other traditions as well...
the „sufferer/looser“ (stemming from joseph’s house) and the „winner“ (stemming from david’s house) have to become one inside you – that’s, in my opinion, the occult meaning of the hanged man – that he recognizes that the world can only be overcome by completely accepting it – taking the cross, the tav... being „in“ time AND „outside“ it. at the same... ah... time [grin]
and, the next arcana, very fitting, are death, temperance, devil, tower, which show that the journey after this moment of enlightenment still leads through the battles of life, until after the big crash (or very possibly more than one) one gets slowly ready for the reality of macrocosm – the stars, the moon, the sun; and then finally the world again (holding two wands, perhaps the two pillars?), but in a creating-dance now (not like the fool who „just“ dances)
...hope i wasn't rambling on too much.
cheers
pali
(p.s.: i like your questions - they always give me reason to think again...)