Thanks for the answers. The pipes from the foundation are terminated with seemingly-soldered adaptors to provide compression threads. One chromed faucet pipe is stuck in one of those, with the compression nut unscrewed. The other chromed faucet pipe has been removed by me (easily) and replaced with a hose designed for connecting to the faucet, and is now leak-free. Except I still might want to connect other stuff to it.
The lower end of that faucet hose has compression hardware built-in, to connect directly to the compression threads mentioned above. Is that kind of hose-end connection reusable? Or does the first-time compression of it cause it to become significantly less reliable the 2nd time you compress it? In other words, can you unscrew it, put a valve or tee or something like that in between, then screw it back, and not expect it to leak?
As for the stuck pipe, if I cut it and bend it, can I "unscrew" it, causing it to become unstuck by lots of turning? Or is there a better way to remove it? It seems like it has something inside catching on the soldered compression adaptor. And I can't put a lot of force on it to remove it, because anything I use as a fulcrum to get leverage, just bends the floor of the under-sink cabinet.
I don't want to destroy the stuck pipe till I'm sure I know how to remove it, because, presently, even though it has a very minor leak, it's working, when I put the compression nut back on. And I have to turn off all the water in the house to work on it, because there is presently no stop valve there. So, if I cut it and bend it, to try to unscrew it, we're stuck with no water till I finish the job.