Well, I've gotten somewhere. I found the IBMDSS01.SYS socket service and IBMDOSCS.SYS card services versions which are supposedly for the 755c with Ricoh chip. This was very interesting because they both loaded just fine. At least they said they did and there were no complaints. On top of that, I was able to add the CAD. After that the packet driver loads just fine!
But it doesn't work. There are different errors when I try do do something. Mostly it relates to not having a MAC address. I can remedy that message by putting one in the PACKET.CFG file, but then other errors occur such as PING saying there is no ethernet address available.
A check on the IRQ says that it is not in use. I've tried several available ones (10, 11, 15) and none work. I think what is happening is that the IBM services are not accessing the card like they should. Could it be the "card services"? Nothing I do makes the card light up.
Other than (apparently) not having the IBM files for a 755c and/or RicohRF5C266 I think I've gotten to the right configuration.
One other problem is that IBM stuff tends to come in image format with lots of files intended to be installed on a HDD. Since I'm only testing, that approach is out of the question. I wish to only work on a 1.44 floppy. There's more than enough space for anything I could possibly want, including diagnostics. Anyway, that's only a minor problem and it just makes IBM a little less user friendly than they could be if they would just make the necessary files available. Perhaps they used to. Now it's Lenovo and a lot of their stuff has "off shore English" which doesn't inspire much confidence.
From what I can tell, the Thinkpads are only similar on the outside, and they all require individual software. I guess nobody has a ThinkPad of the 755c variety or they'd likely pipe up with the correct files.
So far I've spent about 20 hours on this. Plus, it took a month to get the card from eBay and I don't even know if it works. That was a waste of money. At this point I feel really stupid for having even tried this. I think it's time to throw in the towel and admit defeat.
Thanks to those who offered help.