1. On freenode I have a registered nick, but $usermode +r is not set. So I am not sure that this will work reliably.

2. For NICKSERV identification by mIRC as part of the server configuration (as opposed to using a script or perform to do it), then mIRC knows it has requested authentication by NICKSERV and all it needs to do is to wait for a NOTICE of any form from NICKSERV before attempting a join.

It does not need to worry about whether the authentication actually succeeded before attempting a join - if it failed and channel is +r then the join will fail because the authentication failed. If it succeeded then the join will work.

I cannot comment on other authentication methods such as SASL.