Why is my diplexer such a spectacular failure?
I feel that my original question was basically too broad. I'd like to break this diplexer problem into two questions, one concerning how to best use a simulator to analyze an RF circuit such as this diplexer, and another question that addresses best practices for interfacing a circuit on a PCB with coaxial I/O.
I also convoluted things by initially posting a circuit image that was incorrect..I apologize for this.
The Ham.SE community would benefit much more, I feel, from very specific and answerable questions regarding simulation and device construction/assembly.