We had the outflow of the holding tank
blocked this summer as well. First I tried to push an antenna
cable through, this had no effect. Then I put freshwater pressure on the through-hull on the outside. The freshwater hose had to be thickened to get the same size as the through-hull internal diameter.
This worked. The marina got quite dirty.
You can try to dissolve the salts with cleaning
vinegar, which is much stronger than kitchen vinegar.
Backflow from the holding tank: This is basically backflow from the contents of the hose going from heads to holding tank. I tried for some time to prevent this by lots of maintenance
to the valves of the heads. Finally I converted to the newer type of Jabsco
heads. In this model you can use "twist and lock" the pump handle, which prevents backflow. Probably you buy the parts
separately from Jabsco