Thanks for the tip 21. I assume you also need to use the 'Repostion Images' mode in the Photomerge popup using Edit In Merge to Panorama from LR. I moved the middle layer to the bottom and locked it, then did Edit> Auto-Align Layers using 'Perspective' mode. Next I adjusted the images slightly for best alignment, which was already very good, then did Edit> Auto-Blend Layers with 'Seamless Tones & Colors' checked.
It's better, but still has vertical convergence and rectilinear distortion:
