UPDATE: After trying many different things, different boot orders, and doing some diagnostics on the health of all the drives in my desktop computer, I chose to purchase a new SSD (1 TB) to reinstall Windows.
I unplugged the original, small 128GB SSD that came with the computer (and that did not really have anything of value on it), and I plugged the new SSD in there and restarted, with the USB stick Win10 Pro recovery files as the boot-up choice.
It finally installed easily. With my OneDrive subscription, I got back most of my files already because they were accessible on any computer, including the laptop I've been using for the past 10 days.
There are some files that were backed up only on Carbonite and I am in the process of retrieving those. This included many photos and videos, along with my Outlooks .pst files. So, for right now, I have emails dating back only to Oct. 1. But the data is all there in Carbonite--it just takes a while to get back what I still need.
One small annoyance: ALL those files WERE on the 3 TB drive when I first came here (I could see how little free space it currently had). But when it was suggested that I format the one SSD to start over with Windows, ever since then the 3 TB drive was shown as having ALL its space free, as if it had also formatted THAT drive. (No, I did not accidentally choose the wrong drive. The SSD drive DID format, and it was the only drive I chose to format before deciding to just buy a new SSD.)
So, I have no clue how that happened, but it's essentially why I have to go back to Carbonite to restore all those files. It'll mean a lot of boring restoration time, but at least nothing was lost.
This is a cautionary tale to everyone to ALWAYS USE some form of cloud backup!
I'm happy to be back using my main desktop computer again. Thank you for helping me in the earlier stages of this problem. All is currently well again.