My issue with Seveneves is the first part (two parts I guess) was a pretty first-rate thriller then hard to believe transition to equally unlikely and not very interesting IMO outcomes which, as you say, weren't really wrapped up either.
And, even if you accept how they got there, the last part of the book just wasn't very compelling for me.
That last part almost seems like a form of trolling, the smallest viable story to claim "I had you on a sidetrack all the time and you did not even notice"