kwaw, I'm thinking somewhat along the sames lines as you. The solar cross probably symbolizes the Christ. For Waite it might have something to do with the second marriage he mentions. For example, the solar cross in union with the lunar High Priestess. The same symbolism is on the Ace of Cups as the solar cross on the wafer, and the lunar chalice.
Your idea about Tiphareth is interesting. It makes this quote from Waite's Fellowship of the Rosy Cross Zelator ritual a little more clear:
GUIDE OF THE PATHS: In the mystical astronomy of the soul, the moon is like natural reason, a realm of reflected light, peopled by wandering thoughts and plastic images. But we seek, O Honourable Master, the direct illumination at another and eternal source. I am the Reconciler between them, standing in the Middle Path, looking towards the Glorious Sun of Tiphereth, as a purified Moon of Mind, and testifying to the Bond of Union.
In the FRC, the Guide of the Paths symbolizes the He Final and guides the initiate through the Portal grade between Netzach and Tiphareth but no higher.
If the union of High Priestess and the Christ is Tiphareth represents something having to do with the Restoration, that would make the High Priestess a very important card indeed, as Waite mentions.
