Sharkbites may not be perfect but they are way better than cpvc. Nothing is worse than cpvc because it gets brittle and cracks. You should really get it replaced before something bad happens.

This makes me think that the treads were sized correctly so that when they're finished, all the rise heights are the same. Make sure you check your local building code for proper rise/run requirements. There should also be acceptable values for overhang. Most treads that I've seen at the store have a little overhang built in. That way you install the treads, then risers, and everything falls into place. Disclaimer, I haven't actually done this project yet.

Does your house have plaster walls? When they squish the plaster against the lath some usually drips down and makes a pile at the bottom of the stud bay. I recently renovated my bathroom and found every stud bay had 1-2" of plaster piled up in the bottom.