First thing you need to do is make sure you do not exceed the 5v limit on the input pin! Possibly use a 4.7v zener diode to limit the input.
Putting an R across the sensor will not increase its sensitivity. The sensitivity is a function of the design of the sensor.
1) Not a problem with common 1" piezo disks. They don't have the current capability to damage a pin.
The input pin protection diodes clamp the voltage spikes.
2) A piezo is basically a capacitor. Without the resistor there could be DC on the piezo.
A resistor to ground keeps the piezo at zero volt, so you can measure the actual pulse.
1Meg to ground could add to wiring/hum problems.
The voltage spike on a piezo could be positive or negative, depending on how the piezo has been dropped onto the brass disk in the factory. If you're serious about it, you should measure it.
I do this with the piezo resting on an O-ring the size of the piezo, and a DMM on the lowest voltage setting.
I gently/slowly press the center of the piezo (face up) with the red wire of the DMM, while holding the black wire onto the brass.
You either see a positive going voltage or negative going voltage when pressed, and the opposite when released.
You should have a positive going spike when hitting your drum pad, so a piezo with a negative going voltage when pressed with the above method. Assuming you glue the piezo disk with the brass side to the drum pad.
You have a 50/50 chance of getting the right piezo.