Im not a Hvac pro but have done basement remodeling. I dont think it
is worth an argument over placement , you will have circulation. but you
do need them to add more supplys to just get in more heat, and they
agreed to do that.

A thing I have found is on many remodels, basements are cool if
underground completely, on my last one I had to remove and insulate all
the ducts as even with the vents off the basement needed a heater in
summer just from the cold air passing through to the upstairs, I hope
you considered this. Many basements just need a Dehumidifier and the
return open in summer as humidity is the issue when it is warm out.