Got it working... here's what I found for any others searching...
The "Prompting" message seems to be due to Excel generating some sort of error message that is waiting for user response. I found I could just click "OK" on the Prompting message and the selected contacts.csv file would import. (Although there is no further indication of whether it succeeded of failed). I just waited a minute.
TYT CPS 1.26 (and probably previous ver) really do need M$ Excel 2013 or later to work- trust me I tried a lot of other things.
I then connected the USB to the MD-9600 and hit the "Write" button in that same Write Contacts popup window. For this I then got a progress bar and it took around 4 minutes to complete. Then (very important) one must get into Radio Settings and find the "Contacts.csv" menu item and select/turn it on.
The contacts.csv file needs to be comma delimited with the following fields:
Radio ID | Callsign | Full Name | Nickname | City | State | Country
I had to create the Nickname column, but I put no entries in it.
I also eliminated the header row because I suspect it eats up one of the contact slots in the radio's memory.
You will have to edit with Libreoffice Calc (free) somewhat to get the output from DMR-Marc or RadioID.net to conform to this. You'll need to trim the total number of contacts (I believe) to 90,000 or less, because 10,000 records are in the Main Codeplug Memory.
Per Tom's (of N0GSG Contact Manager fame) suggestion, I stripped the Callsign contacts out of the Codeplug Memory/File, and just use that for Group/Talk Group info. It makes things SO much easier to manage. Then use Contact Manager (I highly recommend this to make life easier- it supports several different radios) just for managing Channels and Groups. (great for mildly OCD folks such as myself who prefer things to be in the proper order).
After I sent the modified Codeplug that was outputted from Contact Manager, I found I had to go back into the Radio Settings and re-enable contacts.csv
That's about it in a convoluted nutshell.
Thanks for the helpful pointers
73s