As far as nickserv still sending out notices about the nickname being registered, it's been that way for a long time, and a different network, Epiknet, behaves exactly the same regarding those notices. Also, in the debug, you can see that the identify command is sent at the same second than the notice is received, so it's looks normal for nickserv to be too late to cancel the sending.
As discussed before, this is the notice that mIRC looks to know that it should continue with the perform.
What you quoted doesn't contain a message from nickserv that appears once you're identified, so I'm not sure which notice you're talking about.
In the debug, the join message sent by mIRC is clearly sent before mIRC has seen either the +r usermode or the notice from nickserv which is in this case "[16:51:41] <- :NickServfirstname.lastname@example.org NOTICE Ouims :Password accepted - you are now recognized"
I'm sorry I don't really understand your reply or how exacly is mIRC deciding to go ahead and send the join message, it seems to me that the timestamp shows the same seconds for the join and the nickserv notice that would be used by mIRC, but the join command is displayed first, indicating it was sent before receiving the nickserv's notice
] -> fiery.ca.us.SwiftIRC.net JOIN #mircscripting,#pacman
] <- :NickServemail@example.com NOTICE Ouims :Password accepted - you are now recognized.
[14:39:21] <- :fiery.ca.us.SwiftIRC.net 477 Ouims #mircscripting :You need a registered nick to join that channel.