1。 not unusual for old house to have water damage. If there is no softness when you step on it, it is probably ok.
2. the cut-out and the two little wood block that sit on the 2x8 were done very unprofessionaly (the same for the little piece of wood on the concrete block in 15) and do not appear to be secured by nails or screws. You may have to replace with something stronger but keep the drain pipe because the grade needed for the pipe.
Overall not significant and should be no more than a couple of hundred of dollars.