As a newbie, how am I to know for sure that my instructor is great or the best one to teach me? How would I know the difference if I have no comparison other than the internet? I can find multiple opinions on almost any topic on the internet.
You don't. It sucks, but that's the truth. Reviews, how well you get along with the instructor in person, and word of mouth/recommendations from others is all you can try and make a decision based on.
Next question(s). Does it make sense to take the OW test from one dive shop and the AOW from a different dive shop? Will I learn more because of working with different people and places? Will I get different advice on gear or skills?
Yes, imo it does (or at least a different instructor). Even if your 1st instructor was really good, a new instructor is going to provide you with a different experience (personality, etc.) and potentially some different points of view. This is one great way to learn in my experience - and one of the best ways to learn if a previous instructor you had maybe wasn't as great as you thought they were.
How do I know that I am getting the best instruction available?
Hindsight.... cost doesn't always equate to good or bad instruction, reviews can be wrong, the instructor can have an off-day, etc.
You can generally tell "terrible" instruction (I had an instructor for a specialty tell me 'I can't teach you anything' after dive 1 of a multi-dive specialty once for instance) during the course, but anything short of that you'll figure out later on.
Are there compelling reasons to use different dive shops and instructors if I am happy with the one I am currently working with?
I think there are. Different instructors have different experience to draw from. They have different teaching styles. They have different opinions. They may have different ways of doing the same thing. Getting exposed to those varied opinions, methods, etc. make you a more informed diver and helps you to figure out what your opinions, methods, styles will be moving forward.
I have received training/certifications from 10 different instructors, 8 "shops", and 5 different agencies and I think the exposure to the different people, agencies, and locations for that training helped to make me a better, more informed diver.