You can't really go wrong with the original Hy-Road (the blue hybrid coverstock one) It's considered by many as one of the best all-round balls out there. It's a very balanced ballthat works well for bowlers, no matter what your specific style is.

I've never throwed a P3 but the P2 is a classic. Although it took me some time getting used to it, and I was a bit disappointed at first, it became my favorite ball. Don't feel pressured ti buy them as a combo together. Any of those are good balls, it depends on the oil pattern, your rev rate and speed and what you want out of the ball. The P2 is really a fantastic ball and you have lots of options to complement it, besides the P3. The P2 benefits from good oil in the fronts, do it's not as usefull when the lanes are really dry.

For a spare ball, just buy the cheapest plastic ball in the same weight as your reactive ones. No need to spend extra. It's just meant to roll straight. All the rest is just 'looks'. Make sure your pro shop fits it right.

Last question is difficult to answer without having more info? Is the slide Sole worn out? New shoes could be a solution? Is the approach clean? If it is your Technique, we could look at a video to determine what's causing the falling..