well you're half way there, but I'm still betting you haven't tried your own suggestion... if you set your alternative nickname to be the same as your nickname, then indeed the raw 433 script will kick in. However when the raw 433 script sends the new nick, if it is accepted by the server, then mirc sets the ALTERNATIVE nickname to it, and there is no way to set it back without manually going to options->alternative nickname, etc
There is no way to get the primary nickname using scripts if you're using an "alternative" nick. There is a $anick identifier, however if you're using the alternative nick, then $nick also returns the alternative nick.
If there were something like a $prnick which returned your primary nickname regardless of which you were using, then making a script like you suggested would be FEESIBLE.
Therefore, I believe 3 things should be implemented in the next release...
1. minimally a $prnick type identifier (and /prnick command to set it), and,
2. if the raw 433 script HALTS, then mirc should NOT send the alternate nickname anyway
3. like the original request, an option to have mirc handle "nickname already in use" like irssi