I believe this question was partially answered, since the answer only specifies how to install a theme.
Yes, to install additional mouse cursor (x11) themes for the current user, the theme must be placed in the "~/.icons" directory.
Now to set it, "gnome-tweak-tool" should be installed.
sudo apt-get install gnome-tweak-tool
After having installed gnome-tweak-tool, open it and navigate to Theme, and choose your Cursor theme from there. Please note, setting your cursor theme through there will only change it for certain applications.
Now comes the tricky part, setting the theme without modifying the /usr/share/icons/default/index.theme file.
In your ~/.icons directory, create a directory named "default". Then, inside that directory, create an "index.theme" file. Open it with your preferable text editor, and add the following settings to it:
Inherits=<cursor theme name>
Then log out, and log in again, you should see your new cursor theme appear just for that user account. I believe this is what's going on. When loading a cursor theme, the system would first check for a ~/.icons/default/index.theme file, and if it cannot find it, then it will use the system's version (/usr/share/icons/default/index.theme).