At least according to the Mystery Science Theater 3000 wikia, you are not correct in your pre identification. That source gives the following timeline:
Jann Johnson (1989-1990)
Alexandra Carr (1990-1992)
Mary Jo Pehl (1993-1997)
Beth "Beez" McKeever (1997-1999)
Also, for Beth, her IMDB page under the Actress tab, shows several voices for MST3K ...
According to this article:
In interviews with director Robert Rodriguez, he revealed that the surprise casting is meant to lock in capable actors for big roles in future Alita sequels, which was a lot tougher to pull off than it looks.
Note that the lack of credit might not solely be due to the surprise effect, but also because of a technicality:
Norton makes a lot of cameos vs the number of his actual appearances. Norton in the past has expressed sentimentality to embracing character's "mysteriousness". I believe his cameo this time was to be a dead-ringer for James Cameron and thus went uncredited to add to the illusion.
As you might already know by now it was set up for MonsterVerse franchise based on Japanese Kaiju monsters. And there are a lot of apparently 17. The four we see are:
Godzilla, Rodan, Mothra and Ghidorah.
Godzilla and his powers have been already displayed in Godzilla (2014 film), still to reiterate: