>why is support for more did methods an assumed goal?
Because extensibility is good. New DID methods are constantly being invented and there shouldn't be any artificial restrictions on their use.
>for whom and in which use cases is a non http protocol handler justified?
In an 'http' URL, the authority is derived from the domain name. In our case, authority is derived from a cryptographic identity, so a custom URI scheme is more appropriate.
>why is did:key important?
did:key doesn't depend on any external services and is the easiest to implement.
>why is ap: the best possible url scheme for the AP protocol?
It works and so far no other scheme has been proposed.