There is no such thing. The "default shell" is a decision made by the distributor.
Typically a root user sees the '#' symbol as their prompt. If they already have a prompt via the PS1 shell variable then the # symbol is usually added at the end to indicate that they are a root user in this current context.
Shell is the bridge between Kernel and User.
The Linux administrator is called the "root" user.
You don't need a shell script for this; just use the 'id' command look at the uid, which for root is 0.
The default user shell is set in the password file as an entry for each account.
Yes.
With most Unix-derived systems, "root" is the super-user account.
The root account is the administrative account on Linux; this account has control over everything in the system.
Strictly speaking, a "shell" is simply the interface between the user and the rest of the operating system. In that definition not all shells on Linux are command lines. However, most day-to-day usage of the term "shell" does refer to command line user interfaces. Finally, "Linux shell" is not a thing. Linux is an operating system, specifically a kernel, not a shell. Most Linux distributions use BASH, but I personally prefer ZSH.
No. There is no single user interface for Linux. GNOME. KDE, Xfce, Fluxbox, and CDE are all examples of user interfaces / desktop environments for Linux.
Unlike in MS Windows, where any user can alter, change, or damage the system, only a root user can make such changes with linux type systems. So it is an extra security measure.
By default,there is only one account, root. However, most Linux distribution will either allow or make you set up a second administrative user account.