PIC I2C Clock selection
I'm working with PIC24 but I can try to help you.


It depends on your system, you can choose any clock source to fit your requirements (10kHz-100kHz).You can use Fosc if you use a core frequency in the desired range, or Fosc/4 if needed, you can also use the timers (TMR0/2/4/6) if they run at the right frequency.
MFINTOSC provides two clocks (500kHz and 31.25kHz), but only the 500kHz seems to be mapped to the I2C peripheral. For HFINTOSC, you need to set the register OSCFRQ, but the minimum frequency is 1MHz (not in your requirements).
Yes.
I think it will work with the same clock as the MCU. I2C peripheral can use Fosc clock without dividing it by 16 (not as the UART).