Leveling 80 - 90

[MiRai]

Just some quick general answers to get you started...

a) Is it best to instance? or Quest?

What's "best" is what you find more appealing so that you don't get burned out; but which do you enjoy more: The monotonous scenery of instances over and over and over again, or the outside world where you can breathe fresh air?

Most people mix it up.

b) Is a hunter the best thing to boost with? (Probably no, thus what is?)

Any class/spec that does burst AoE damage and can either soak up incoming damage or kill fast enough to avoid it.

c) What is the best build to boost with?

I realize you're asking about your Hunter, but I answered this above.

e) Do I just do 5 man questing and level all of my accounts at the same time rather than pulling them up with my 90?

This question sounds like your first one... just rephrased.

f) Once this team hits 90, which of the classes do I want to use to pull the rest of them up?

Any class/spec that does burst AoE damage.