That definitely sounds like a hardware problem then, but you also said "Two different repair companies have tried three different replacement LCD and digitizers. The problem remained." Are you using a screen protector, or a case with a screen protector built in? When you take it off, what happens to the problem?
Here's a way to more definitively verify that it's a hardware problem. Temporarily change the keyboard layout to something like DVORAK instead of QWERTY. What happens? Do j and n start working, but now the two keys where j and in were (with QWERTY) no longer work?
A similar test would be to change SwiftKey's settings so that the keyboard is detached from the bottom of the screen, allowing the keyboard to be moved freely to anywhere on the screen. To do this, first set your keyboard layout back to QWERTY. Then, bring up the SwiftKeyboard and press and hold the "123" button for a few seconds. Choose undock, move the keyboard to a new location, and then test each key.
These two tests could effectively show whether it is a hardware problem or a software problem. I'm leaning towards hardware based on your last comment, but these tests should add further proof.
If it's a hardware problem, one way to show the repair people where the problem area (the area not responding to touch) truly is would be to get install some kind of drawing app. Something that will let you freehand paint on the screen with a finger or stylus. Using a small brush size and one solid color, try to paint the entire screen. It should then be pretty obvious where the screen is responding to touch and where it isn't.
One possibility that could make it a software problem: After installing a new ROM, do you restore data from a backup? Have you tried not doing that?