Merp. I'll try to answer your questions as best I can.
Re: MechE vs ChemE. I think you're right. I remember there being many more opportunities for MechE. I remember the ChemE guy struggling a bit. He took on a huge engineering load.
Re: Internship. There's a pairing mechanism not unlike med school residency pairings. I believe the algorithm was built by a student. From what I remember, you rank your choices, and the partner companies rank you. The best combinations win. You can apply to any internship you're qualified for. For me, I was a systems engineer, so there's no way I could take something in ChemE, for example. There are a boatload of opportunities for MechE.
Re: Regrets or things I wish I knew. I'm personally struggling a lot with my student loans. Even with the nice fellowships we receive, I started the next chapter of my life $160 K in debt. That's a significant lean on your sense of freedom. Nothing seems quite as fun when you feel like you HAVE to work. I feel the grip loosening a bit.
Also, you will feel separate from the Sloan crowd. You will try to fight it, but it's somewhat inevitable, though a couple people were exceptions. This was a positive and...not negative, but you get the idea. Having a tight group of 50 people (there's a bunch of us planning a pirate ship cruise in Seattle for Labor Day weekend) is...well...what life is all about. LGO is a crucible. It will change you.
Re: Surprises. Maybe that I was accepted? I didn't consider myself a strong candidate going in. Though my confidence grew, I was neither an engineer or an alum of a top university (I was University of Washington, which is great, but isn't Princeton). By the time I submitted my application, I was so committed that I knew I would receive an acceptance. Get there. Spend time on campus. Show up to classes. Talk to students. Get tons of feedback on your application. If your brain goes to "that sounds like a lot of work," rather than "tell me more ways I can get a leg up!" It may be a slog.
Know WHY you want to get into MIT.