I liked the first part (technically first 2 parts) of Seveneves. But then [POTENTIAL SOMEWHAT SPOILER] it felt like vastly unlikely events come to pass and Stephenson spends the back part of the book telling the completely different story he wanted to tell.

yeah, hence the "mostly" in my comment. FWIW I really liked the whole thing, but 100% agree it's almost like 2 loosely-related works that ship together. It's the 1st (longer) part that I thought "felt" realistic given the premise.

Red Mars is exceptionally slow though. I made it through the first novel, but I had to stop somewhere a quarter into the second because there was just nothing interesting happening (or maybe too much of it?).

That's very much a matter of taste though - I loved the pacing of the Mars trilogy and the related "Antarctica" which is pretty much White Mars.