I sincerely thank you all for your suggestions and recommendations. I've saved quite a bit of the detail in the messages for another trip.
To put an end to this story, I did manage to get the Holiday Inn Express at the Frankfurt airport at their Point Break rate. Of course I didn't have enough points in my account so had to purchase some.
It was a bit of a cliff hanger. I learned there is a max 48-hour waiting period for points to be posted once I'd made the purchase. Tried to call and have somebody help me do the transaction all at once, but HI was taking a well-earned holiday break. I bought the points via computer the morning of December 24. Kept checking and the hotel was still there but points weren't. Finally, Christmas morning the points posted but the hotel was now off. Fortunately, when I called them they now were at work and were able to still let me have the hotel even though it no longer showed available via my computer.
Mainz is definitely on my target list for another visit though, and I've even passed the info on to a son who has a Germany visit planned.
Also, I think I'll take the S-Bahn to Mainz this trip, just to visit.
And I'll be watching that HI Points Break list for other trips.
Romelle