Thanks for the insights. Unfortunately I can not remove the ductwork (it would entail lots of work considering the logistic of its postition, etc), but there is not much ductwork to worry about so it should not take too much time.
I live in a condo unit so I share the walls (concrecte block and bricks) and it also happen to very close to nature and the trees/florest, so pests can be a problem (but I haven't had any since I started to close and seal all possible avenues of entrance that I could find). Such avenues of entrance were mainly spots where pipes (sewers lines, natural gas, water) and the ductwork meet the walls, in such places I used insulating foam, mortite, and steel wool (coarse 3) in order to insulate and also bock the entrance of pests and bugs, and no pest made its way into the basement so far. Since I have aluminum windows in other parts of the unit and no real windows in the basement, the basment is now the warmed part of the home (and it is not due to air leaking off the ductwork).