Mystery Fruit Solved!
It was driving me crazy that I couldn't figure out this fruit. And I couldn't believe that nobody else knew what it was either. Of coure, once I went out and inspected the tree again, I realized my clues sucked. I don't know how I missed the thorns on this tree the first time and I totally remembered the leaf configuration wrong. You may have to click on the picture to enlarge it to appreciate the thorns. They're huge and the tree is covered with them.
As I said on Facebook, I was afraid to knock on the neighbor's door and just ask. When I moved in here, my landlady told me they were very cranky old people who hated this house because of a dispute over the driveway boundary. Well, after I took the shots and picked up a couple more fruit, they happened to outside, so I screwed up my courage and just approached them. Boy was my landlady wrong.
The woman was sweet as anything. She hugged me immediately. Unfortunately she didn't know what the blessed tree was either. Told me it had been gifted to her by a friend and that they grow wild out in the country here. But she explained how she turned the fruit into decorations for the holidays and invited to help myself.
As it turns out my brilliant sister's suggestion that I cut the fruit in half is what finally tipped the scales. Damn if it wasn't a citrus fruit after all. Don't know why the skin is a little fuzzy, but once I split it, it was clear it was an orange of some kind. And the seeds looked different when I cut it instead of just squishing it by stepping on it. Cleary citrus seeds, as you may be able to see in this picture.
Thus armed, I hit the google and finally identified it as a Japanese Hardy Orange Tree, also known as a bitter orange and sometimes referred to as a Flying Dragon, I assume because of the thorns.
Apparently the fruit is edible but very bitter. They suggest it can used to make a juice though, if you sweeten it. Here's what a healthy tree looks like and if you care about the botanical info, it's here and here. Thanks to everyone who tried to help.