#1 Fix the return issue.
Agreed.

#2 Both gas lines need a drip leg.
I have no idea what that is.

#3 Electrical run needs to be secured.
What electrical runs? The only electric in the picture is in the
light blue flexible conduit to the HVAC and the black cable on the
right that runs down the sump pump PVC to the sump pump itself. The
blue thing near the water heater *was* a loose water line to the water
purification system, which was put in place after the picture was
taken.

#4 I would remove the PVC from the water heater relief valve.
No. It's not physically attached and there is room for overflow if
the PVC gets blocked. It's tested weekly. The pressure valve goes
out and I end up with a flooded basement again. This directs it to
the sump pump. The PVC is rated to handle the temperatures involved.

#5 Install a media air filter.
Thank you for the suggestion.