I’m not, but equally don’t assume that the people in the Python packaging community have any knowledge of LibreOffice. Generally, as soon as a Python installation stops looking like a “standard” installation (in this case, you don’t have pip installed in the LibreOffice installation of Python, and you therefore need to use
--prefix to do the install) you’re likely to be on your own, sorry.
Picking up on some specific points you asked:
No idea, that’s entirely down to the LibreOffice folks. They could do any one of a number of things, and there’s no way of knowing without looking at a LibreOffice installation.
If you’re not using a version of pip installed into LibreOffice, that’s expected.
Basically you’re in unsupported territory at this point. The supported answer is “install pip into the LO Python environment and use that”. You can also use
pip install --prefix or
pip install --target, but those are highly dependent on the target Python’s expectations, and you’re back to “it depends what the LO install expects”.
That’s very far from “fully catered for pip”. To fully cater, they would need to include pip in their Python site-packages and provide a way for you to invoke it, like a standard Python install does. Can you invoke the LO python from the command line, even? If so, then
python -m pip should give you a running pip, unless the LO python doesn’t include pip.
Can you not use that to install Python packages then? If it’s a wrapper round pip, it seems possible that it would be usable that way.
Sorry, that’s about all I can offer. You’ll need to do some more investigation and digging on your own, I think. Maybe if you can come up with some more specific questions that can be explained without relying on people knowing about LO, you’ll be able to get additional answers.