Quick Answers
Dependent
As many as possible
Dependent
Explanations
If you are using insecure, public methods, it's probably better just to run stealth, because you are probably going to get banned reguardless. Also if you are using a big farm, you can save a lot of resources by using stealth. On the other hand if you are using a small farm, or if you are using a private method or just have an account you are willing to bot but want it to last a few bits longer because it takes a while to design, then it's probably better to use mirror. using stealth on top of private methods also won't be super harmful either, it just stacks faith on top of security.
Simple. A farm is a farm. The more production, the more gold, the more gold the more profit. the mo' profit the mo' bitches.
Different gold farming methods yield different results. Anything public is more likely to get you banned, such things as green dragons, blast furnace, etc. Especially the public ones that either give high xp, high gold, or both. Private things such as _______ do not get banned nearly as much and i make so much fucking money off of it right now that I have to buy 5 banks so i can put my money in them.
(Get it? Because private.)