The unit itself is only three years old.Sooo I decided to spring for all new ductwork and vents - they also widened the tiny 2 x 12 vents in the larger rooms for better air flow - it took two full days and cost $3900 for a 1600 sq. ft. house.

If they only said seems like low air flow and increased the speed that is not a proper evaluation. 

Originally this was to facilitate warm air flow from 1FL to 2FL I think.

It sounds like you can pay to treat the outside of the property with some kind of chemical spray that keeps the bugs away, but I've heard that spray will quickly wash away and lose its effectiveness6) Crawl space - open the vents in the summer and close in the winter to promote air flow in warm weather and keep heat in during cold weather.

They can't charge for basic wear and tear - aka the paint brushes and spackle and cleaning supplies (unless there were holes in the walls or some damage you caused).They can't prove you caused the mold in the bathroom, unless they have before and after pictures blatantly showing it, you still should have cleaned it to the best of your ability, but even so, mold is cause by lack of airflow, generally the landlords problem.They only have 30 days to return your deposit or the remainder of deposit and a list of what was charged out of it - any thing over 30 days, you get your money back, regardless of actual damage.

Your units might need to have duct cleaning and servicing or your units are old and can't properly give adequate air flow to the space.

One other thing to check, make sure there is a few inches of clearance at the BOTTOM of your baseboard heater, you need to have airflow through these units.  

The supply lines are at floor level under the cabinet & I've used Jedd's idea of loosely skirting with good air flow in front of the back of the box with pond liner. 

So I am pretty certain  that this lack of " air flow" is contributing to the problem. 

If there is a crawl space, it's worth inspecting the perimeter of the home for any vents, gaps, cracks in the foundation that would allow air flow - grab some straw and lay it over these gaps to keep the wind from blowing in.