"Any" help comes:
Just put it into rc.local and forget the rc.inet1 stuff.
wpa_supplicant -B -Dwext -iwlan0 -c/etc/wpa_supplicant.conf
What is it good for? Do you have any GUI programs that manage your connection and rely on rc.inet1.conf?
You may even replace rc.inet1 with your own, eight lines long one and get rid of that very long script. Beware:
do not forget to set up loopback interface ;-) This may lead to strange behaviors, I can tell you ;-)
The idea of Slackware is to know Linux, not any particular distro. If you know how to set up wpa_supplicant, why learning how to set up wpa_supplicant via Slackware startup script?