Thanks to Dave for suggesting two things:
1) Before I get totally frustrated about the Captivate NOT being unable to be seen by my main (Windows XP) Desktop as a drive (for copying files), why not try it with Windows 7?
I connected the phone via USB to my Win 7 laptop, and it just worked. Not only was I able to copy an Excel file to the phone, but, when I opened the 'QuickOffice' software on the phone, it instantly saw the Excel file and opened it as normal. This is a big thing.
2) Before I get totally frustrated about issues syncing with my USA.NET email account, and having the UCWeb browser not able to do a Send when accessing that account via WAP, why not just attach to my gmail account (that I have automatically picking up USA.NET mail). That worked great for offline reading, and the UCWeb browser displays gmail wonderfully, when connected via WiFi.
One gmail feature worked great on mail sent from the phone, that I was sure would fail. When I send mail from gmail on a computer, I want the sender to show up as 'email@example.com', my primary mail service. I sent mail to myself from gmail on the phone, and, sure enough, the sender was stamped as I want, not as my gmail address. This was, for once, a pleasant surprise.
So, after a lot of teeth-grinding and imprecation-muttering, I popped my sim back into the Captivate, and will work with it for a few days.
Now I need to find out how to get Android to see the MP3 file, that I copied to the phone, as a selection as my ring-tone. How hard can it be?
All absolutely-critical functions are now working, and I can tinker with getting my XP computer to recognize the Captivate at my leisure, of which you must be thinking I apparently have far too much.