I'll run 4.5 feet of beer line from the faucets to the kegs.
This might not be long enough. You have to balance your system, and in my experience the stated psi/foot is often too low. So don't go cutting anything just yet.
QUESTION - does the gas line lenght matter like the beer hose lenght? Can I run a 2 foot gas line from the CO2 tank to the manifold?
As in no, gas line length doesn't matter, and yes, you can run a 2 foot length from the tank to manifold, and from the manifold to the kegs. Use however much you need to make it easy to connect/disconnect kegs.
Real world QUESTIONS - Once I put beer in a keg, what do I do? Seal the top, then what? Should I just attach the gas line and leave the gas at at 10 pounds until in comes out?
Seal it and put pressure on it. Minimum of 5 psi to seal the keg. How high you go depends on the carbonation level you want in your beer and the temperature of the beer. The volumes of CO2 desired in a beer is typically determined by the style. You can find some values here
or several other places, that's just the first one I googled. When you know your temperature and desired volumes of CO2 you can look it up in a carbonation chart like this one
, again the first one I googled. I highly recommend carbonating at serving pressure, it takes a little longer but is really easy with no danger of overshooting on the carb.
Do I dial down the pressure after the beer is carbonated in a few days?
I wouldn't, just leave it at serving temperature/pressure and it will equilibrate to the right level of carbonation. Then cut your serving hoses to the proper length for the psi you are serving your beer with.
The other option of course, is to carbonate it to the proper level, then adjust the gas while you're pouring to get a nice pour. Since you're using a manifold instead of a bank of secondary regulators, this is the most realistic option if you're trying to serve beers with different CO2 levels. Just remember to turn the gas in each keg back up to the carbonation pressure when you're done pouring at your serving pressure.