0.5) Yes. There will be two totally independent users that happen to have the same password. This option just saves you from typing the same password multiple times. You get the same option when entering the password for the database installation if you remember.
1) Yes. ASM and DB have totally independent users. The SYS users in each are unrelated.
2) No. The users don't have to have the same passwords.
3) The password is changed by altering the passwordfile. Remember, the ASM instance doesn't have any datafiles of its own, so it can't store passwords internally. It only uses the passwordfie for this. See MOS Note. ID [452076.1] here: https://support.oracle.com/CSP/main/art ... d=452076.1
4) EM uses preferred credentials to access the database and ASM instances. You typically have to enter database, ASM and host credentials for all the functionality to work. As long as you change the preferred credentials for the ASM instance, you should see no problem.