[cknuth]

My turn to have a Cesana adventure has come around.  Thanks to all who have gone before for the excellent resources.    Here's the report:
Rough Colorado roads shook loose a number of things, including my shower door secured in the closed position (now I know better.)    We stopped to investigate banging in the back and found the door hanging from the flexible track; the top curved tube was lying in the shower.    Checked in with Newell and https://www.ebay.com/usr/bagnoericambi?ul_noapp=true.    The latter offered the two replacement joints (P/N MRN072298AS,) including shipping from Italy at about half the Newell price.    And they arrived in less than a week.
Removing the stubs of the broken joints required impact as noted.    It took a lot less force after I drilled out the dimples or stakes that secured the splines in the top tubes - see photos.    I tapped and added a 1/4-20" set screw to each side to replace the drilled out stake.  This made the side connections more solid than original.    Fitting the middle curve back between the two joints required "flexing" the sides of the stall.  Nothing was bent or broken, but I wouldn't want to push it any further.    After tightening all fasteners and reinstalling the top monorail, the door functions better than ever.    I suspect the joints had begun to crack long ago, as the door had sagged a bit.