You and a million others want to start making money with their hobby, photography. So I would not get my hopes up. That said, it’s more of a business undertaking than a photography undertaking, and every genre is different. So start with what your of photography business genre you are interested in… then you might get some useful advice.
The easiest fit a beginner is going to be Lightroom because it is the most widely used by magnitudes, and that means there is unending help and tutorials. You go the road less traveled and there’s little help available outside a small circle of users.
I always said in the day that a 400 2.8 lens on a monopod hanging over my shoulder, and another body and lens on the other shoulder, was an instant press pass. I walked into a few football games and right down to the field without anyone questioning me. I don’t think that would work for NFL, but it works for the lower leagues sometimes.
Sounds like you had a falling out with someone at your old school about the yearbook and now you are talking about lawyers. Really? I would say to be the bigger person. Your photos likely don’t have any financial value. You were planning to donate them to the yearbook before the falling out. This is for your alma mater. It is for the students. I would forget about it, or send them something that says they have permission to use your pictures in the yearbook but nothing else. Maybe you could ask for a credit mention, but it’s likely to be a mess and life is too short. Be happy that the real benefit is people enjoying your work.
Not sure what you have committed to, but I think you will almost certainly have to use flash if there isn’t some unusual lighting going on. Parades are moving. It is dark. I would not think they have lighting even as good as a high school gym for indoor sports. The lighting is the key. Have you used flash before? You will need to understand how it works at distance. You will need to understand flash sync speeds, etc. You really need to do some testing beforehand, but I doubt you can recreate exactly what a parade will entail… slow moving vehicles will people standing on it moving around… bands marching down the street, etc.