Once I got home I had a better chance to analyze the problem.
The latch mechanism on my case was always very stiff to operate, and when I pulled on red handle to open the case it dislocated the inner tab on the handle from the slot it is supposed to move up and down in. This prevented the case from opening.
It was possible to unlock the lid by instead pressing down on the inside edges of the rotating cams under the red cover. With the case open I was then able to remove and disassemble the locking mechanism.
The silders that move back and forth to unlock the corners of the case have a lot of friction, making it very difficult for the rotating cams to move them back and forth. I removed the sliders and filed them down so they move freely. Now when I unlock the red handle, the springs on it pop it open all the way and fully unlock the lid, with no need to even lift the handle.
This took about 10 minutes, so if anyone else has a balky top case handle, you might try this.