I guess, if you maybe were to assume that on a physical level and not on the meta world level, that the Golden Land is the gold, the 'river' could be a reference to the fact that if Eva did find the gold through a tunnel in the well, that there is, or was water running through said tunnel. And if the tunnels are extensive enough, they might reach beneath the mansion.
If battler couldnt lift the door to the well... how eva and rosa who have an occupied hand for the gun would lift it up?
I haven't come up with anything solid yet regarding the riddle, but I'm trying a few different angles.
Spoiler for A few things about the riddle:
For one thing, here are a few weird things about the riddle.
1.) It gives you the command to "follow downstream" twice, in the second and third line.
2.) The First twilight and Second Twilight. For one thing, the First Twilight is the only one to mention the key. Then the Second Twilight says "The surviving shall tear apart the two". I suppose this could mean to split some word into two different words, or perhaps add something to the beginning and ending.
3.) The Third Twilight. The third twilight has nothing to do with killings, and it seems really out of place in the sacrifices. So, you probably get some important hint after you complete the first and second twilight, and you have to figure it out before you can continue.
As for the few angles I'm working at:
里 is made out of "two close radicals", 田土, or if you could also extend the 土's top and say it's made of 日土.
Another angle, from the same line:
I am also wondering if the Kanji is misleading you into reading it differently. For example, rather than 里, maybe you should be looking at その, which not only means "that" but it can also mean "garden", in other words, the garden outside the mansion. I can't really think what "two things" in the garden could point at, but I think that it somehow leads to the church. Maybe if the Arbor and Storage are facing in a way where, if you "drew a line" straight from them, they would give you a "point" which is the shore.
A different angle:
Sweetfish could be refering to his children, who left his "beloved hometown" I.E. Rokkenjima and went out into the ocean and had children. Another way to get to this is 鮎 = あい = 愛 so, 愛の川, which could point out "a family", and then the two who tell you of the shore are Kinzo and his wife.
Rokkenjima's shores are probably the waters of the Pacific Ocean, 太平洋(たいへいよう), which has six characters. Perhaps the First Twilight is "getting these letters" and the second twilight is splitting them into "たい" and "へいよう" or "たいへい" and "よう".
Perhaps the second twilight points you somewhere, as hinted by the third twilight, and then, following the other twilights, you would kill "5", leaving one letter. If that letter is い, it could point to the well.
It could also be much like a game, where you can only take out one letter if the remaining letters will still form a word, for example:
Right, I'm quite new here, sorry if I make some mistakes...
Anyway, I was thinking things through regarding the epitaph and the golden witch, and realized something:
Spoiler for The epitaph of Umineko no naku koro ni:
Gold with a questionable background... a witch of the gold who has somehow survived for more than a thousand years... the one who solves the epitaph and finds the gold becomes the new golden witch... the philosopher's stone?!
Quite farfetched maybe... maybe the epitaph is a riddle how to make the stone, achieve the ability to make gold out of nothing (like the creator-witches), and become the ulitmate golden witch?
At least that would fit into the category of western magic.