If its not T&G subfloor then they likely used cdx and didn't see much glue in the one photo.
Blocking would be a solution, but not possible with a finished ceiling below.
Replacing the SF would not be something I would not be interested in.
Stairs, unless you have changed something, adding 3/4" to the top rise creates a awkward and potentially unacceptable variance in the rises. 3/8" is typically maximum acceptable variation. People base the rise by the first step.
I assume any staris leaving this area is going to a lower level? It really is the same deal if you are dealing with ascending stairs, the rise is less, however if someone trips they fall into the stairs not down.
PVC pipe would not be my choice, but certainly acceptable.
End matched refers to the ends of pieces of flooring also being T&G, pretty common and necessary if using random lenghts.
Why are you set on using a true hardwood floor?
Depending on where you live the joints can open/ close seasonally.
If I was looking at this job for a client I would be steering them to LVP floor, apply some good underlayment, replace the baseboard and move on.
