The impedance of the resistor divider is approximately the same as the resistor from analog input to ground; this resistor is in parallel with the other as far as impedance goes (look up Thévenin equivalent for the reason why). Since one resistor is much larger then the other, the smaller one predominates.

Isolation may mean many things. I think a more useful term here would be "Protection." You want your PIC to survive if something goes wrong, something to prevent 100VDC from being applied directly to a PIC pin.

I agree you need this, having done such myself. The PIC will not survive. However, most of the pieces stay on the table for a further failure analysis.

I would do two things: Put a zener to ground or a diode to VCC at the input to work as a clamp. Plus, I would split the resistor from 100V to PIC into 2 or three resistors. Resistors tend to fail open, but probes and such can short them anyway. By putting another resistors in series you can still short one out and have the next one provide some impedance for the protection diode to use. Without such impedance the protection diode will open followed shortly by the PIC.