1. Yes. How else would a database be able to keep track of its objects?
2. In this case, you could. You can use the same password - you just keep track of the SID when you open an MS-DOS window. See #3.
3. See below. Yes.
Value/Description
NONE
Setting this parameter to NONE causes Oracle to behave as if the password file does not exist. That is, no privileged connections are allowed over non-secure connections. NONE is the default value for this parameter.

EXCLUSIVE
An EXCLUSIVE password file can be used with only one database. Only an EXCLUSIVE file can contain the names of users other than SYS. Using an EXCLUSIVE password file allows you to grant SYSDBA and SYSOPER system privileges to individual users and have them connect as themselves.

SHARED
A SHARED password file can be used by multiple databases. However, the only user recognized by a SHARED password file is SYS. You cannot add users to a SHARED password file. All users needing SYSDBA or SYSOPER system privileges must connect using the same name, SYS, and password. This option is useful if you have a single DBA administering multiple databases.