Pulling 4 inch flex duct through a tight space

I was wondering if anyone had any tips as to how to pull 4 inch ductwork through a tight space. The duct to my bathroom has no air flow and the duct is crushed, so my plan is to put a "T" in my bedroom line. Unfortunately im in a condo and dont have access to the 15 inch space between the floors.

Basically i only have access to the two register openings and about 10 feet to pull the line ... ive been struggling with this all day... any thoughts on how i can get the line through?

 

There will very likely be negative consequences to splicing the two vents together.

I would never install flex duct if there were any way to put in rigid.

Without seeing what is there, it's pretty hard to comment on how I might do it.