Just to follow up on this string for anyone who encounters a similar problem:
1. I pulled the lock tumbler out by drilling a deck screw straight into it. Then used a large sponge and a hunting rifle mount I happened to have in the back of my pickup as a fulcrum against the door below the lockset. Used a pry-bar and easily popped the lock tumbler out. No damage to the door or lockset.
2. I used a small flat-head screwdriver and lifted up the brass vertical shaft that locks the bolt into the unlocked position. "Up" is unlocked.
3. I used a claw-end screwdriver and gently inserted it underneath the bottom right corner of the KT lockset housing. I lifted up the corner, which broke off at an old crackline which I had noticed years ago and painted over. This created a very small access hole right over the bolt.
4. I inserted the claw-end screwdriver and was able to easily pull the bolt back, then pull the door open. The bolt is spring-loaded, of course, so I had to hold it in the open position while pulling the door open.
5. The inside door lever was functional, so I pulled the lever up and taped it to the interior of the door to keep the bolt from snapping back into place.
I will now need to remove the KT lockset and fix the problem, which I anticipate being a snapped lever on the inside of the flap handle which pulls the bolt back.
Thanks, TG Twinkie, for your advice and for reaching out to me via email. I appreciate it, and I look forward to staying in touch.