We are also working under the impression that your Ebay refractometer reads in brix. Some are designed to be used for antifreeze or salt water aquariums.
I would also recomend getting a refractometer. I use this chart from More Beer to check my FG. I check with a hydrometer every now and then but so far the chart seems to be close enough.
The only thing that comes to mind is that a small leak may not be enough to show up on the flow meter. An easy way to check is hook up an empty keg, put 20 psi on it and then close the valve on the CO2 tank. Check in an hour. If the regulator still shows 20 psi you are leak free.
look at the cap on the bottle. Twist offs usually have little arrows pointing to the right. You can also see if the crimp mark is parallel around the circumference or if it slopes up to the right. My fiancée thinks I'm nuts when I check the bottles of a beer I haven't purchased before.
I ferment in my corny kegs. Can't break them, they are light tight and can rack to serving keg with C02. As stated above, trash the cracked carboy. Not only is it a injury waiting to happen, the crack could potentially harbor bacteria.
I think thats a hard question to answer. Different buffing wheels wear down at different rates depending on the material being buffed and the compound being used, as well as how aggressive the user is. How many kegs do you need to do? Chances are that you will need to replace the wheels...
Welcome to HBT from a fellow Seattleite. +1 for Brouwers and Naked City. Have you read read the Naked City thread? Also Bottleworks in the Wallingford area is a great place to buy beer.