Re: 22 pitch on 150 merc
See: Almost no one will answer that question without a photo because stresses on any prop are high. Without seeing if the broken spline has caused a crack (almost invisible or not) we must err on the side of caution. Additionally, we need to see if it is a solid hub chopper or a thru-hub exhaust prop.
However, to answer your last question: A broken spline, being at the center of the prop will have very little effect on balance, if any.