How was his f-i-l able to live before Reb Elchonon married his daughter? I don’t mean this facetiously but isn’t it possible that the f-i-l was at an advanced age and just happened to die at that time?
To answer the first question, maybe so that he could support his R’ Yitzchak Elchonon in the future, and maybe because his time wasn’t yet up.
To answer your second question, of course anything’s possible, a story isn’t an iron-clad proof. The timing was pretty good though, and nobody is creating a brand new idea, that Torah is “eitz chaim hee lamachazikim bah” (a tree of life for those who support it)* from a story. Rather, the story fits right in with and illustrates this basic concept in Yiddishkeit.
*Yes, I know that’s a drash, and the simple pshat is “for those who grasp onto it”.