That is strange, I have never had it happen in a Mustang with manual locks, it has always been the actuator binding.
May sound dumb, But have a buddy beat on the outside of the door by the latch assy while you pull on the inside handle. I have had to do this on several Explorers to ge tthe door to open for service.
What happens on the Explorers and I am sure on the Mustang is that latch gets dirt and crap in it, and over time that the shaft inside the latch starts to get play in it. It allows the lock tab to bind at an angle, causing the door to not open.
If beating on it does not work, I am not sure what to tell you. No real way to get the latch apart with it in the door.