Correct 500mA vs 1000mA would be twice as long.
Yes, when the light turns green the battery is "fully charged" I haven't tried to charge via my USB port, wait till it turns green, then plug it into the wall adapter to see if it keeps charging or not.. but IMO the battery seems to last longer when charged with the wall adapter. Could be me, I dunno.
I've noticed the same thing when charging my Ipod via my USB port vs the Wall adapter for it.
It could be all in my head but I swear it seems like it lasts a lot longer.
As for a cheap USB cable.. Personally this phone is worth too much to me to risk shorting it out or bricking it with a cheap cable. I'd only use cables made by HTC for this phone..
On the other hand, the phone's charging port is a standard mini-USB connector so as long as the cable / charger is putting out at 5V / 1A it _should_ be ok.. Again, personally I'd only use cables made by HTC.