A lot of questions there, lol, but first, just leave Quiet 'n Cool enabled because yes, it does help keep the temps lower by allowing the CPU to downclck when idle (with the Balanced power plan).
Second, unlocking is hit or miss. I don't remember the actual percentage of CPU's that successfully unlock, but it's nowhere near a certainty. That's why I said earlier in the thread that if someone wants a quad core, they should just buy a quad core. Spending less money on an X3 is nice and all, but there's no need to take the chance that you may or may not be able to successfully unlock the 4th core, just to save money.
Third, if your temps are OK in Prime, and they are, you're good. No real world usage will ever push the CPU as hard as Prime does.
Fourth, Temp sensors are different for different mobo brands, so I can't tell you exactly which "TMP" sensor is right for your mobo, but just look at the core temps and "Package" under the CPU section, and don't worry about what's listed under the temperature section. What I outlined below is what you want to look at.