I've some questions about cold backup & initpw & orapwd tools

Oracle under UNIX
in init.ora : remote_login_passwordfile=EXCLUSIVE
select * from v$PWFILE_USERS is empty

Monday :

1. under unix, I restore a cold backup (cp -p xxx xxx) : all datafiles, all ctrl files, all redo log files
2. startup my db :

sqlplus internal
(ps: that 2 commands work fine BEFORE restore) :(

Total System Global Area 124083596 bytes
Fixed Size 64908 bytes
Variable Size 62046208 bytes
Database Buffers 61440000 bytes
Redo Buffers 532480 bytes

ORA-01991: invalid password file 'orapwxxxxx'(BEFORE restore, I NEVER received this error message!)

in fact,my db is mounted but not open!

So, I tape : 'alter database open;'
then, my db is sucessfully opened (but strange, starting needs 2 steps ???)


In doc 'Oracle 8i (815) Administrator's Guide " I found :

ORAPWD tools


This parameter sets the password for SYSOPER and SYSDBA.
If you issue the ALTER USER command to change the password after connecting to the database,
both the password stored in the data dictionary and the password stored in the password file are updated.
The INTERNAL user is supported for backwards compatibility only. This parameter is mandatory.
To Replace a Password File

Note which users have SYSDBA or SYSOPER privileges by querying the V$PWFILE_USERS view.
Shut down the database.
Delete the existing password file.
Follow the instructions for creating a new password file using the ORAPWD utility in "Using ORAPWD".
Be sure to set the ENTRIES parameter to a sufficiently large number.

and in the other hand, I found in some forums something like :
' alter user or orapwd is the same but orapwd is safest and orapwd needs shutdown'

OK, first I try :

3. alter user sys identified by yyy;

but always same error during starting db ('ORA-01991: invalid password file'), SO I suppose that password file is NOT
updated (but the DOC SAYS IT SHOULD BE !???)

4. second I try orapwd tools (shut + orapwd + start), and it works
sqlplus internal
work fine again

My 3 questions are:

A. WHY after my restore, the password file is become INVALID ?????
B. WHAT did/not did I do ? tommorrow, if I restore again my backup, will have the same password file problem ???? A restore has it a link with the password file?
C. Why alter user DON'T UPDATED my passwod file

Thanks a lot for your help