Some users have recently had their accounts hijacked. It seems that the now defunct EVGA forums might have compromised your password there and seems many are using the same PW here. We would suggest you UPDATE YOUR PASSWORD and TURN ON 2FA for your account here to further secure it. None of the compromised accounts had 2FA turned on.
Once you have enabled 2FA, your account will be updated soon to show a badge, letting other members know that you use 2FA to protect your account. This should be beneficial for everyone that uses FSFT.
Seems unlikely they would be so low unless you live in Michigan or something. Depending on cooler, you are unlikely to get lower than 5C-10C above ambient on air. Adding 15C to that would put you at 32C, which seems a more reasonable idle temp for an overclocked E6750.
It may be misreading. I wouldn't worry about it unless your HSF is super hot to the touch and its really misreading. there are people on here who run extreme cooling and have their rigs running in the negative temp range.
Due to the way the C2D sensor works....CoreTemp (or any other program for that matter) does not always interpret the digital signal properly. The BETTER way to use CoreTemp is to check the option that says something along the lines of "show temp delta to TMax" which makes it show the temp delta, before throttling will occur.
Trying to interpret actual temp is a hard thing to do and can vary quite a bit from revision to revision, but using the delta to tmax is much more accurate.