Sometimes when my connection is particularly bad, my two previous nicks will both not have been registered by the servers as being disconnected yet, so when mIRC tries to automatically re-connect, it gets stuck at the server demanding that I use a different nick. So I don't get logged in unless I happen to notice this and bother to type in a /nick command.

It would be better if mIRC could store more than one alternate nick, and/or have an optional automatic pattern defined such as adding more and more _'s or ''s or whatever until a nick works.

Also, while I'm on the subject, I'm always bothered by the way mIRC sets my primary vs. alternate nick.. it seems that if I use my alternate nick, even if mIRC uses it automatically, it then becomes my primary until I bother to change it.. and even figuring out how to change it isn't that easy. (I don't remember what the issue was, I guess that you can't change that mIRC setting while you're connected).

I wish mIRC would just *keep* my defined primary nick as my primary and my alternate as my alternate, unless and until I define it differently.

Not a bad suggestion but I think this should be part of what has been asked many times in the past, a way to define servers to join automatically, with nicknames set per server etc..
mIRC doesn't change your main nickname unless you use /nick to change it yourself, I believe. However there is an option to keep it from doing that, check 'preserve nicknames' in alt + o > Connect > options

The simplest solution to this would be to increment a counter and append/replace it to the end of the nickname in conflict.

i.e.
Nickname
Nickname1
Nickname2
etc...

This design actually replaces the alternative nickname function completely. Though you could increment w/ the alternative nickname instead of the original.