as being a former 'professional' window tinter as you call them. there is no real easy way out on old tint
just get a box of razors.. if you have a steamer use it!one of the best ways for removing tint... the heat helps the glue stick to the tint, and can then just be pulled off.
as for the defrosters.. if you're in a place where you really need them.. be really careful.. take your time.. but more than likely.. they are already toast, and only being held there by the tint.
I would try a small section, a top or bottom line or something.. but if the tint is old as you described.. sad but true, may aswell just scrape away with the razors.
The tint shops that are saying 'you gotta tint all the windows' are just being stupid.I used to retint single windows all the time.only thing is, it is not going to match the rest of the windows when you get it tinted.. but you might not be able to notice the difference too much if the other tint is in good shape and not too faded..
goodluck with it.
-Dan