As i explained, u dont need to use ADC for frequency measurement. Simply use method 1 i described above. I used same method many times to detect frequency, and its simple.
All u need is a comparator and capture of CCP module; and u r done.
use comparator to convert sinusoidal wave to square wave
send the square wave to capture module, initialize capture module to start counting on positive edge, if capture counting expires, maintain a static variable in interrupt routine, increment it, until another rising edge occurs. multiply the max count of capture into static variable plus the current count and use that count to divide the counter frequency, the result is ur answer of frequency.