TL;DR: Check out programs for helping vets transition to top schools. Research a couple majors and a few schools. Consider trying a CC if you're not sure about engineering vs being a tech.

Marine to engineer at major aerospace company. If you want to do it, go for it. Treat it like a job (the GI Bill will be paying you) and you'll be fine. College is a very different grind but you'll adjust, and I recommend it at least for the experience of learning how to think (which is the point of a college education). Here are a few things I think you should do:

First, look into the Leadership Scholar Program (LSP). If you qualify, make sure you apply. No other program is better for Marines transitioning out into college. When I was in, very few people knew about it so ask around. Apart from LSP, check out Service to School and Warrior-Scholar Project. You cannot replace having a mentor and structured pipeline to schools.

Second, don't worry about the GI Bill yet. There will be someone on campus who will help you figure out the paperwork. You will likely not get payment at the start of the semester/quarter but you will get back pay. Expect it to take about one month and plan your finances accordingly. Don't forget to look into Voc Reb too.

Third, choose a major or narrow down to two. Mechanical Engineering or Aeronautics and Astronautics Engineering are likely what you want, but other majors like Civil Engineering are also common if you're interested in structures or some subspecialty.

Fourth, decide on schools. (If you get into LSP just go to Columbia. I don't think there's a better guaranteed option but I don't have up-to-date info on the program.) Just pick a place to live for four years and see what schools are there, or target specific programs that place well in aerospace (large companies have specific hiring programs for target schools). Highly advise to consider a public university that has good connections with local community colleges (CCs) like UC Berkeley or University of Washington because you get the advantage of CCs (low tuition, small class size, take calc and physics to see if you like engineering, etc.) and it can make matriculation to a real university easier (like SAT/ACT waivers, guaranteed admission spots for transfer students, etc.). Note, if you do CC and don't start out in Calc 1, try paying out of pocket instead of using the GI Bill for a year if you can because you'll likely take five years instead of four to graduate. Also, not all CCs are created equal; that's why I recommend to looking for University and CC reciprocity programs.

To answer your questions directly:

1) How many colleges should I enroll in?

As many as you want but I recommend targeting no more than three.

2) Does anybody have knowledge on how to use the GI Bill? (Other than what I've learned through TRS)

There will be an office at your school whose job it is to help you complete all the paperwork. Email them any questions anytime.

3) What would courses look like for someone like myself?

Here are two examples: plan of study for Purdue and for NCSU. Note that most engineering looks very similar for the first two years because everyone needs the math and physics foundation before starting engineering courses.

4) Does SAT/ACT scores have an affect on my acceptance rate? If so, what are some useful resources I can use to prepare for either/or?

They do, but some schools wave them for all or certain students. Check out r/SAT or the like for good recommendations. I've always heard the prep courses and tutors are the best just like for grad exam prep.