達磨山への道
Wow, what a wonderful story.
I really felt immersed in climbing the mountain with them; the wind and the 熊笹 being the only thing you can hear as you climb and climb… It was so interesting to see 常野 interpreted here as (if I’m not mistaken) a clan/large family of people instead of the country that’s seemed to be hinted at so far.
I was wondering at first if 泰彦’s father might’ve been 常野 himself or a descendant of one, and his trip to the mountain was more of a family tradition type thing, but as the details came to light, it seemed pretty clear that wasn’t the case. He had returned with his son in an attempt to see that shard of the future one more time… I wonder if you need two people there for some reason for it to work? 泰彦 questioned why his father brought him at all last time, and every time someone’s seen something, there were two people there. (We only have a sample size of two, granted.) Also very possible he wanted to pass the knowledge of the mountain onto his son as well. 
I’m not all that familiar with the 万葉集, so I’m afraid possible intended meanings of his father poring over it and other collections of poetry goes over my head. Glancing at the Wikipedia article for it, I see
The Man’yōshū comprises more than 4,500 waka poems in 20 volumes, and is broadly divided into three genres: Zoka, songs at banquets and trips; Somonka, songs about love between men and women; and Banka, songs to mourn the death of people.
Which stands out to me as a potential mirror of the life journeys both father and son take here. The poem we’re given I had to look up as well. I didn’t realize it was from the 万葉集 at first and was really wondering if I was tired enough that standard Japanese was becoming incomprehensible to me. 
ささの葉はみ山もさやにさやげどもわれは妹思ふ別れ来ぬれば
A breakdown in modern language is given here:
笹の葉は、この山にさやさやと(心乱せというように)風に吹かれて音を立てているけれど、私は妻のことを一筋に思っています。別れてきてしまったので。
Which yeah, seems pretty legit for both father and son, so a great pick here.
Regarding the ending… I was wondering as I read if he was going to consider murdering his friend to avoid the glimpsed future, and if he did so what would happen. (Given 泰彦’s personality, it didn’t seem entirely unreasonable.) If this is the kind of future vision where it’s already set in stone, that could mean 克也 and 藍子 have already gotten together at this point, unbeknownst to 泰彦。 After fully finishing the story though, I think I’m leaning more on the side of suicide. Or perhaps… If we consider that 泰彦 only cites part of the poem, specifically
さの葉はみ山もさやにさやげども
at the end and then says he can no longer hear anything, perhaps it’s not an indication of suicide, but of his acceptance of the future he guesses at. 
Anyway, great to read others’ thoughts on this story; there was a lot to dig into, and it’s always so much fun to see what stood out to others.