BPHAT writes:

In my opinion, they need to remedy the duct situation, refund the
service calls, and replace the unit.

There is a very common type of contractor that may or may not do the work
correctly, but who has the highest expertise in weaseling out of any
warranties, far beyond your amateur abilities to prosecute.

The best approach I've found in such situations is to find another
contractor who will take pride in fixing the errors of the first.