When flying in uncontrolled airspace, which ATC frequency should I contact if I have an emergency? Question 1: If I am flying in uncontrolled airspace and I have an emergency, should I report on 121.5MHz if no ATC is within line of sight communication (which means I cannot hear the ATC and the ATC cannot hear me)? 

Question 2:
Which frequency should I use to report the emergency in the following scenario? When the emergency happens, the aircraft is in uncontrolled airspace but near or approaching controlled airspace. There is an ATC station which is in charge of that airspace and the aircraft is within the line of sight of ATC (which means I may hear them), but my onboard radio coverage (the green circle indicated in the picture) cannot reach the ATC station (which means ATC cannot hear me). Should I report on the ATC frequency as maybe some aircraft are flying in the area and can relay the emergency to notify the ATC? Or should I just keep reporting on the common radio frequency like 121.5MHz?
The safest approach is to use 121.5 until someone instructs you to try another frequency.  ATC will monitor 121.5 at all locations it has antennas at. Additionally, military, some airlines, some medvac, and other operations will monitor 121.5  So your coverage will be greatest if you just use 121.5, unless you are already talking to ATC.

Addendum: Really in uncontrolled airspace?    If you are really in uncontrolled airspace, in the CONUS, you are likely flying rather low.  121.5 will be your best best in that case as well.  With enough time, you might work out a relay with higher traffic, perhaps on a center frequency.  But 121.5 remains the go to frequency.