What I've said over and over, I'll say again, not that I'm an expert. Far from it. I'm not even in the Coast Guard. I'm JUST a mom that wants the best for my son's ... and everone else's (daughter's too)
Choose the rate that YOU WANT to do for a career. Don't settle for anything but what you want. If it's what you want, it will be worth the wait. What if you went OS and ended up hating it? What if you went to your first duty station and tried out a bunch of rates (shadowing), saw them in action and then chose something else? The result is that you would go to a-school well-informed and knowing that you did your homework hand's on, not just by reading a write-up about a rate, or chose the school with the shortest list. Choose without reguard to the a-school list. You will be happier in the long run with your choice.
And as far as switching after two years??? I'm not too sure about that. Why would the CG spend all that money to train you in a job, then do it again because you wanted to?
Yes, you will graduate a-school as an E4, or Petty Officer 3rd.
FYI, my son started at the bottom of the ME list in January. He is now just about in the middle.