There is only one way to eliminate the rust and stop 2" of water from sitting in there.
You must keep the vent to stop misting up in the car and keep air flow ventilation.
Cut open the pillar then set up and cut short the vent so no water ever sits inside again re weld  and re seal
I have done this on many mk2 Escorts
These images are to a 4 door I am currently doing.
Hope this helps you to understand the solving problem
Jim
 

Thanks for the pics, had my hand up in there a few times, but actually seeing it helps.... Good to see someone engineering a solution to a common problem......
Cheers Drew......