Really quick answers to what I know. If Joe Johnson happens across here, I hope I'm not embarassed too badly.
1. Without getting too complicated, 20 PA teeth are thicker and somewhat stronger. Noisier and more prone to backlash as well.
2. Helical gears are stronger, quieter, and more efficient when well lubed because the gear gradually engages. They're also more expensive, and develop a thrust load that you have to counter with good bearings.
3. High efficiency comes from using the fewest gear downs possible, mostly. You lose efficiency from the gear teeth meshing, bearings, etc. You lose a LOT if the gears are too close and bind. Of course if they're too far, it's noisier and you have more backlash.
4. After it's built, you can measure power input vs power output. There's a team I can't remember that will test your drivetrain for you.
5. Tolerances. The tighter your tolerances, the less backlash, the less noise, the higher the efficiency. But it's tolerances that matter. If you have bad tolerance control and try to cut the center distances too close, you can end up binding the gears. Why do you care how quiet it is? It will scarcely drown out the music on the field...
6. Assuming 0 tolerance, add the pitch diameters and divide by 2. In practice, you can add .002" to allow for machining and it should cover you well.
6.1 I've no idea what you're asking... if you want to know a sliding shaft fit...
ISO hole/shaft fits are metric fits. I highly recommend getting your hands on any copy of Machinery's Handbook. A paper thickness would leave you with a whole lot of play.
7. Ball bearings? Flanged might make assembly easier, but the choices are limited. Unflanged means you have to worry about press fitting or otherwise capturing the bearings.
7.1 Sealed are well sealed againt water, dust, etc. with rubber typically. The seal holds grease in the bearing so they don't need maintenence.
7.2 Sheiled have metal shields to hold in grease and keep out the worst of any debris. Also maintenence free.
7.3 Open are just that. the balls and races are exposed to air. you'll probably need to grease them occasionally, and the balls and races could get dirty. If you're running very high RPMs you could use oil in open bearings instead of grease.
7.1,2,3 Other than that, they're effectively the same for FIRST purposes in terms of torque, etc.
Again, you may want to invest in a copy of
Machinery's Handbook either hardcopy or on CD. It's terribly useful for all sorts of things. Anyways, I'm gonna give my poor fingers a rest now....