1. Are SpeedFan readings reliable? Is my CPU really 21°C?
Who knows? It seems a bit too much low, at anyway.
Give a look to that temp with either Core Temp and Open Hardware Monitor (or similar tools).
2. Are AMD's "Cool 'n' Quiet! Technology" + ASUS Q-Fan the best that can be achieved, and I won't get a more silent fan by adding a 4-pin fan controller like a FanMate?
Apart the Zalman FanMate doesn't have a 4-pin connector (but the more costly FanMate PWM does, if in case), why can't you simply use SpeedFan to dial down your fans?
Generally speaking, a really good (the FanMate isn't so, it's an above average unit) hardware fan controller is always better than any BIOS control, even the best ones, in order to silencing a rig.