The book does do a much better job of conveying both the timescale and the complexity of the problems he's solving, but it's hard to show that in the film without totally losing the audience.
There were a couple of disasters left out from the film and I thought that made it way more believable (if a little less interesting). The capture of Mark was a bit silly and in the book the blowing of the airlock was enough to reduce the closing speed.
I did like the ending though. That bit of closure was something I thought was missing from the book. Knowing that Mark goes on to continue his work with NASA etc.
As to Sean Bean's casting. His character is not a 'rocket scientist', he's a crew administrator. Basically a manager that looks out for the rights of all crew members and understands the psychology of people on these missions. For someone to have experience in that they're likely to be an ex-military type. His resignation is not called for in the book though, him and Teddy have a discussion where he basically knows there's no proof and they want to keep the 'mutiny' a secret so he knows he can't be done for it or there'll be questions.