@Karifean: I think judging the difficulty is going to be difficult for non-japanese people (or at least people without knowledge of the language). Without knowledge of the Japanese language the epitaph certainly is unfairly difficult.
Alright, I will just address my own thoughts towards the end of the epitaph. While it might be just meant as a dedication, as WitchofGames said, I tried to see whether more can be read into it.
The witch shall praise the wise and bestow four treasures.
One shall be the gold from the Golden Land.
One shall be the resurrection of all the dead souls.
One shall be the resurrection of the love that was lost.
One shall be to put the witch to sleep for all time.
Sleep peacefully, my beloved witch, Beatrice.[/quote]
a) What did Kinzo think when he wrote those lines (since it isn’t part of the riddle, he could’ve added this later)?
From what I remember, Kinzo suspected that the child from Beatrice 2 was still alive and closeby. So under that assumption I will look at the four treasures.
Treasure 1, the gold: obvious
Treasure 2 & 3, the resurrection of the dead and of the lost love: Kinzo wanted a person that he could project his regrets regarding his deeds towards Beatrice 2 unto. Thus he wanted his fifth child to serve as a stand-in for Beatrice (resurrection of the dead) and project his love unto that person as well (resurrection of the lost love). A very selfish request, as soon as Yasu solved the epitaph, Beatrice’s dress was forced on her to serve as the “resurrected lost love”.
Treasure 4, to put the witch to sleep: However, Kinzo wants only one thing, to express his regret to “his lost love” and thus put those unresolved feelings to rest. In a way, the witch Beatrice is the personification of his regrets (so Beatrice is a being in his mind, similar to the Kinzo in Natsuhi’s mind in EP1) and by gaining the chance to apologize, he can put the Beatrice in his mind to rest.
Sleep Peacefully: Consequence of the fourth treasure.
While all of this is incredibly selfish of him, his interaction seemed to me as if he would’ve left Yasu alone after saying his piece (even had he not died immediately after) and putting the Beatrice in his mind to rest. He wanted to say sorry, that is all. But as it happened so many times in the story, things were forced upon another person without regard for his or her circumstances.
This kind of mirrors the magic ending as well. Tohya also wanted to put the Battler in his mind to rest, and Ange eventually gave him the chance. Although it is no where near as destructive as what Kinzo did to Yasu.
b) How did it influence Yasu?
Probably not much of a question, of course it laid the foundation to the “lore” that would eventually be integrated into the Legend of the Witch. Although it could be questioned whether Yasu had figured out all of the “lore” regarding the Golden Land, or whether it is more a product of the meta-world and developed during the course of the story itself (Usagi could surely say more about this aspect in general).
c) Can certain episodes be attributed in their themes to a certain treasure?
More a random thought. EP7 and EP8 are pretty clear that they are about laying the witch to rest, thematically speaking EP6 is about the lost love, the gold is found in EP3 and EP5. Probably nothing.