How exactly does the ROTC pipeline work? Will being prior service help me any?

You're degree discipline won't matter as much as getting good grades and doing well academically. I commissioned into AV with a degree in Architecture. My wife is also an AV officer and got her degree in Advertising/Journalism.

ROTC rates everyone regardless of prior service on what they did while they were in college/ROTC. Your grades, participation, awards, volunteer hours, extra-curriculars, sports, anything you did while in ROTC is quantified down into a score that ranks you on a nationwide order of merit list (OML) against every other ROTC cadet commissioning in that fiscal year. The number one cadet gets first pick and so on down the line, so if you're halfway down and they're out of AV slots, then you're out of luck.

Being prior service is definitely an advantage in that you will be given more opportunities to mentor and help the cadets who are straight from high school with no prior service/experience (like I was) but not in terms of actual raw score for the OML. If you want an active duty spot as an Aviation officer, do well in school, do well in ROTC, and get yourself as high on that OML as possible. Rule of thumb: If you finish top 10% you will most likely get your first choice of branch.

On the other hand, should you go National Guard/Reserve, you need to contact the individual states or Army Reserve around your junior/senior year and schedule a board with them where they look at your records, prior aviation experience, SIFT score, PT score, GPA, things of that nature to decide whether to accept you into aviation. OML has very little to do with it. But once you're in, it's the same career track as active duty. I myself went the National Guard route because 1. I wanted a civilian career and 2. I didn't want to chance myself with the OML. I graduated when the Army was downsizing, and AV slots were few and far between.

Hope this helps! Good luck!

The most important thing to do is find schools with the area of study you are seeking. Then find the ones with an ROTC program. Generally, you can do this by region. You do have an advantage over average cadet militarily, but you may be disadvantaged because you are probably out the habit of studying every day.
Advice: Visit the school and the ROTC unit. Talk with the military instructors what do I have to do to become a pilot. Talk to several. Then develop a plan. Determine the standards you must meet to achieve your goal. Work hard to meet the standards.

That's what I did, although that was not my initial plan.
Being a vet does help, since you'll have practical experience while most of your classmates will not. Helped me standout in summer camp.
I'm going to go against the grain here and say, pick the college that offers what you want in a degree. Then check on the ROTC programs.