So I thought WPA was working perfectly. But then it started dropping out on me every now and then. I can't find a pattern to it. But I did leave the SSH connection to the router open on my desktop and the error message "wl0: Invalid argument" was displayed repeatedly.
Sometimes the connection get reestablished after a while. Sometimes I get impatient and reboot the router.
So I gave the instructions for WEP setup on OpenWRT Kamikaze one more try, except this time instead of using my old WEP key I followed all the steps and generated a new key as instructed. And now 128-bit WEP works! I'll have to see if it's stable but I'm hopeful.
Also, "Encryption key:<too big>" still appears when I run "iwconfig". Maybe 128-bit key is too big to display in what they allocated for the field to use? I dunno.