We're now thinking to remove all subfloor and sister the joists to remove the sag. Any thoughts/suggestions? Are we on the right track? Thanks.

Rip it all back to the joists and start over by determining if your floor is stiff and strong enough to handle what you want to put on it. You may need to sister the joist to both increase stiffness and level the floor. Then, after you level the joist, install new 3/4" subfloor and assess height to existing floors to see what the next layer should be.

I don't recommend this - but if you are broke - you could try and put a piece of rubber (like cut from an inner tube, and use a hose clamp - like they use on cars. You might be able to stop the leak. They also have shark bite fittings (which I don't like) but they are code. You need to cut the PEX cleanly and square to the pipe. You could probably rent the special tools you need for PEX repair. How to repair PEX Is it the hot water line? I say this because, some years ago I think Nibco had a problem with PEX and hot water. Typically PEX doesn't spring pin holes.

Probably a mud floor? "Mud" is a lean cement mixture, roughly 5:1 sand to cement. It'll be hard like "concrete", but it is much more friable, much easier to break out than true concrete. Along the perimeter of the floor, is that a waterproofing membrane that is turned up at the walls? Mud floors can be repaired. Clean well, get rid of any loose debris, use a bonding agent, and reshape the slope. Without getting down to a clean base, and without using a bonding agent, the repairs can sometimes separate from the old work. So it may be prudent to break the entire thing out and start over. That would allow you to incorporate a new drain as well. Depends on how far you want to take it and the skill set of the person doing the repairs.