How is the password file accessed when it's on an ASM diskgroup and required to connect to ASM?
ASM metadata describes the location of the password file on the ASM disk. With that information, even you can read it directly from the disk, with the help of kfed (which is installed with the Oracle binaries) and OS specific tools for reading the disk directly, for example dd, without using ASM. Same goes for the SPFILE (since 11g).
More about this topic:
ASM spfile in a disk group
The ASM password directory
And you do not need a password file to start the ASM instance, that is needed only for remote connections.