I also don’t understand the question, “why is the story called kamtza bar kamtza, since kamtza had nothing to do with it”.
Possibly as a memory aid. I think one is more likely to remember the “story of Kamtza and Bar-Kamtza” (especially when the story centers around a mistake made regarding the similarity of their names) rather than “the story of <insert party-maker’s name here> and Bar-Kamtza.”
The Wolf