update: well this more direct rote worked, though I don't understand WHY it worked when the other routes did not:
i returned the book from the overdrive app. then I went to my library using the tablet browser and checked it out again, then went to the
my loans page and selected download epub.
I geta
warning 
verdrive app needed to authorise on this device: confirm YES
then I get a list of apps that can perfomr the "open" and instead of chooseing overdrive I choose bookari. ( which I had previously given my adobe ID to ) . it opened and is readable in bookari and is showing the correct loan length.
so I am guessing the book has to be bound to a specific app when 1st downloaded and thereafter it will open only in that app. i wonder what will happen if I now sync it to bookari cloud and try to read it in bookari on my other tablet....
anyway - this was library to tablet - with no PC in the loop -and no need to buy the KA1 to makje ti happen so I guess its a result of sorts
Meanwhile overdrive replied to say "we don't support helping you to get overdrive to work with other apps" -blah blah. so its a fragmented ecosystem in which overdrive, adobe, app makers can all blame each other and duck any responsibility for making stuff work- maybe they studied UK railways for how to do it
