Assuming you are using 5V VDD and 5V as the ADC reference:
1. Use a 10:1 potential divider. If the current into the divider has to be very low (=high value resistors) follow it with a buffer amplifier.
2. Depends on the response time you need but simplest method is to convert the AC to DC and use a potential divider for the voltage. For the frequency, square the waveform and count clock cycles between level transistions.
3. 20MHz is outside the range of T1CKI according to the data sheet, you might find it works on some PIC samples but not others. The method is to measure T1CKI cycles using T1 as a counter and see how much it increments within a period decided by one of the other timers.
Brian.