(1) If money is not a big concern, I would definitely intern for a federal judge. Its a great opportunity to see the legal system from behind the scenes and to make great connections early in your career. Hopefully you'll get alot of face time with the Judge too, but every chamber is different. I've heard horror stories of clerks who were afraid to talk to their Judges and never got do discuss their recommendations. If this is the case, you may be better off making big money at a law firm. My judge, however, was an overall great guy. I got to discuss every motion with him and we talked all the time. Through him, I got to meet local attorneys and politicians too. You should ask the Judge what the summer experience will be like on a day-to-day level.
(2) I doubt a federal judge would let you split the summer but it never hurts to ask. But law firms are generally good about letting students split summers.
(3) A law clerks' job really depends on the judge he/she is working for. Each judge has his own system for dealing with motions and trials. When I clerked during 1L summer, I read the motions, researched the law, wrote bench memos with my recommendations, and discussed and defended my recommendations to the Judge before each motion hearing. It was an awesome experience. I'm also clerking for the same judge after graduation so it worked out really well.
From an employer's perspective, a summer federal internship looks good on the resume but I still think grades matter the most. You also have to consider that if you land a 1L summer associate job, you'll probably get an offer from the law firm as long as you don't get hammered at a recruiting event and yack on the partner.
One thing I would caution against is working for corporate counsel during 1L summer. Corp counsel rarely hires students out of law school and in the rare cases that they do, they won't pay very much.