OK let's start from the beginning.
XBMC you already know, was the original media centre on XBox. That became Kodi due to Microsoft not being so comfortable with the XBOX reference in the name, and also as it broadened out to multiple other platforms. So the "true" evolution of XBMC is Kodi, not OSMC.
On the Pi Kodi can be installed in three main ways (there are others like XBian, but to keep on topic), under Raspbian, in LibreElec and in OSMC:
- Under Raspbian - installed via the normal apt-get. Not officially maintained by Team Kodi, but by users and fans based on the Kodi source code.
- LibreElec - a package distribution. Basically LE is a bare-bones operating system that is "just enough for Kodi". It comes with only one app installed (Kodi) which boots directly. The OS is very compact and locked down (although it supports Docker) and is focussed on providing a media centre experience that's simple to set up and focussed. Initial Pi4 version available, but still somewhat under fine-tuning.
- OSMC - evolved from RaspBMC. Similar to LE in that it's an OS and Kodi distribution package, but the OS underneath is less locked down and it is possible to install other packages into it via apt-get. But it does not have a desktop, and the main focus is still Kodi. No Pi4 version currently available, but due imminently.
All three are basically ways to run Kodi on the Pi, and Kodi itself can run various addons to provide access to internet-based media and streams as well as locally stored files.
To answer your questions:
4) They have slightly different aims in what they provide. They are not competing, if a user has a use-case where another option may be better it's common that this is recommended.
5) They do work together.
6) To some degree it's a case of what the user is most comfortable with, and what exactly they want. If they only want a media player, then LE is probably best, although OSMC is an option too. If they want more than just a media player, or need other stuff running in the background to support it then OSMC may be better. If they want the full Pi desktop experience with all the other usable apps then Raspbian is the way.
7) That's more limited by the device than the flavour of Kodi used.
8) Again Kodi (which is common to all three solutions) can do this if the packages exist.
9) That would discount OSMC for the moment, but probably not for too much longer.
Personally I run LE 9.2 on a Pi4/4GB and it works fine, although my media is just a mix of SD/HD, no 4K stuff yet.
In the end though all three are available for free, so I'd recommend (using a new uSD card) try all three and see which fits your need and works the best for you. Some people like the "feel" of LE more than OSMC, for others it's the other way around for example.
Also for full disclosure, I'm a member of Team Kodi.