Trivia: The secret project created to use the Hermes to return to Mars to rescue Watney was called Project Elrond, a reference from the Lord of the Rings (also used in the original book of The Martian). Mitch Henderson, played by Sean Bean, was an attendee at the Project Elrond meeting. Sean Bean also played Boromir, who was an attendee at the Council of Elrond in the LOTR movie.
Trivia: When discussing "Operation Elrond", the Director says he wants his codename to be "Glorfindel." This shows he has read the book, as Glorfindel was cut out of the movie and replaced with Arwen.
Answer: The crew (and mission control) are in constant contact with the previously dropped MAV and would have aborted the mission and continued back to Earth had the MAV become inoperable before their arrival. Weir states this explicitly early in the book.
Answer: Yes, it is very risky unless the new Ares MAV is in an area with much calmer weather patterns.