Then you’ll get an error message. 
No. But it is always possible to split off parts of the system onto other partitions/drives. I’ve written a hands-on tutorial about that too, but it’s not for the faint of heart. 
You cannot split off software from the main system at random because all software is integrated into the operating system itself.
However, an experienced system administrator could install the system in such a way that different parts of the operating system are distributed across multiple partitions/drives, or even to have some of it on a server in the network and imported by every workstation on said network.
To the end-user, it would all still look the same, because /home is always going to be /home, regardless of on what physical medium it exists.
Please take a look at the Filesystem Hierarchy Standard, which explains the logic behind the different directories and what they contain.
Also please bear in mind that you are never going to be able to use the system efficiently by holding onto the paradigms of Microsoft Windows. UNIX is a standardized operating system architecture, while Microsoft pretty much created its own island, where things are very different to the rest of the world.
Yes, but that would take several months.