There's a lot of questions here, I'll try to answer what I can.
1. Everyone goes in as an 11B to begin with. Everyone does Recruit Training, and Everyone does OSUT. So, no matter what you end up doing, you start out as an Infantryman.
2. If there's a spot available in the 101st, your Recruit Training instructor will let you know, just be sure to ask. Then you can elect to sign up for that. If there's not a spot available, you can always request a transfer if a spot opens up. It's usually first come first serve for open spots, so just keep your eyes up.
2b. If there's no spot open, after you graduate OSUT, you'll be placed in a unit as a fireteam member, and from there you do ground stuff. They're pretty flexible from what I've heard on the fireteam level, but perhaps one of the Team or Squad Leaders can clarify that.
2c. Medic and Bradley Crewman are a bit more specialized, and those require extra training and open slots.
2d. If you sign up as a pilot and there's not an opening, you won't get thrown out of the 506th, and you won't get locked into something forever and ever. There are individuals here who have transferred all over the place, and while you do lose some rank and time if you do it too often, you'll have some flexibility.
3. If there's an Aviation opening, and you've completed OSUT, and you've reported in, and set up training times, and done your 8ish hours of flight training, and a bunch of other stuff, you'll be a WO1 in Delta Co. Once you've got that, you'll be qualified on all Rotary Airframes the 101st uses (UH-60, AH-6, CH-47, and AH-64). That means, once the op comes around, it'll be up to your Section Leader to decide who goes in what. Most pilots will have flown everything by their 3rd or 4th op, so you will get to fly and gun the Apache eventually.
4. This 101st D/Co. is strictly for the 506th. They participate in every op, once a week (soon to be twice a week with Saturday Ops starting soon), with two ops running at a time, one section (out of 4) for each op. So, if you're a Rotary Pilot, you're required to fly in an Official Op twice a month. A few pilots do more than that, if say one section has an absent pilot, and they need a fill in, or if they participate in unofficial ops (which there are a ton of).
4b. As a pilot, you are required to fly in two operations a month. BUT, for the ops you're not on, you can fill in as Infantry, if they need them. So, if flying gets a little dull, you can often jump into some boots and run around.
5*. If you're not sure about what you'll choose, but are definitely interested in the 506th, go through the the training, sign up as whatever, and try to get in on some unofficial ops. Obviously, they'll be quite different than what you'll be doing on Sundays, but it's a good way to spend some time with some great, trained 506th guys and get a feel for the roles and procedures.
I hope that clears some things up. Good luck!
