Taiwan. You have to figure that out from Natsuhi's flashback on her honeymoon in Episode 5, at the time I didn't think that scene was important at all. 'Areca nuts' and 'some Asian country' help narrow it down, but a number countries fit the bill. I think you also need to know that Taiwan was once a Japanese colony from 1895 to 1945 (making it possible for it to be Kinzo's hometown).
Damshui (freshwater river). I did get that, but not until Beatrice mentioned 6 major rivers of Taiwan.
The Damshui Line. I couldn't imagine railways 'flowing' and got stuck.
. If I looked at a map of the Damshui Line, and got the Japanese version of the epitaph, then maybe I could get to in a week.
Quadrillion. I wouldn't get this in a thousand years whatever language it is in. Qilian is a simple Chinese translation but I wouldn't know what to do with it without quadrillion.
The rest is rather straightforward. Although, you don't have to solve anything beyond the 3rd twilight, which would confuse those who got this far.
Overall the first step is the hardest. Once you get past that barrier I feel you have a good chance of reaching the end.