From a consumer POV, I don't think you're missing out by not being a 3M installer. 3M can sometimes comes off brown, blue green, etc. I like Xpel. The Xpel is more just charcoal neutral IMO. But like anything in life, colorways is subjective.
Yes, start there. A long time ago my area was protected. Now about everyone is on to other products and I don't know if it's even enforced.
They may require a minimum first purchase and want to know that you have good install practices. If you take their install cla$$ you may get priority.
Some of the major brand distributors might ask for an exclusive from smaller shops. Bigger shops or shops that really know their craft may not be asked for the same. It all depends on how bad they want you to have their products.
Having a big brand named product might be a selling point for some but often comes with big brand pricing. The flip side is that you can get marketing support from them to help in promotion. This applies to just about all of the majors in the industry.