I found that I liked air flow through the house and accomplished it with just the fan of the furnace but I didn't have to use the AC.

To solve the problem when people want to close their bedroom door when they are sleeping we cut one inch off the bottom of the door, install a vent in the wall above the door so air flows into the hallway and we even cut holes in the ceiling and install registers with filter so the warm moist air rising to the ceiling can flow into the attic.

May need to replace the indoor fan capacitor on the one with low airflow.

Do you have any troubleshooting or tuning tips to diagnose and address why some of the registers are having weak to no air flow?

It may be more economical to just put a 100w lamp in there, or redirect some heated airflow.. one or both will likely be needed either way, tenant or not.At this point I am just trying to ponder who would be a potential tenant - staying at a furnished place at the lake village, during winter months only… some nerdy person seeking solitude?

If they rusted out, the carbon monoxide and other combustion fumes will get into the airflow and poison people. 

However, there is no way that the air handler is happy with the current extremely restricted air flow--while at the same time, the air flow to Bedroom 1 is way too high. 

Look into replacing your ducts snd running a new truck and return up to the upper floors for better air flow.

When the filter gets plugged, it puts more resistance on air flow.

There is a short-sale house with mildew on all the walls due to the house not having any airflow for quite some time.