One the first issue (with the problematic mentor):
Have you or anyone else ever talked to this person about the way he was communicating with others? I find that many mentors (definitely including myself!) sometime forget that we aren't dealing with grown adult professionals when we work with our students (and my students should take that as a compliment!). In the professional world, people sometimes communicate in a way that can seem blunt or cruel. Swearing sometimes (let's be honest - frequently) happens in engineering/manufacturing environments - from tiny job shops to fortune 500 companies. It's unfortunate, but professionals eventually get thick skin and learn not to take things personally.
A good team leader sometimes needs to remind even the best mentors that we are dealing with high school students here
The message might be even more effective coming from a student.
On the second issue:
Every FIRST team has to deal with this from time to time. There are kids who have honest-to-goodness interest in robotics, but just can't seem to prevent themselves from doing zilch and making others fall into the same habit. There are other students who seem to think robotics is a social club. Putting hard requirements on attendance necessary to travel with the team to competition (and actually enforcing them) goes a long way towards discouraging this. Moreover, good mentors know when you are there but not working. But it sounds like neither the former idea nor the latter ability is something embraced by your mentor.
Have you considered addressing this student during a work session, with your teammates present? Simply make it be known that you are here to do a job, and that this person needs to do his part or go home. Don't be rude and don't yell. But be serious. I have found that when other students call our those who are detrimental to the team, it is far more successful than when an adult does it.