I was finally able to program from Simulink! I cannot explain what was wrong or how these steps solved it, but hopefully this helps someone else:
I followed the advice in that second link, to test the USB connection. Flashing HelloWorld.elf via S32/default flash algorithm did not yield expected results (garbage would print after flashing, and pressing Reset button resulted in not printing anything), but after this, I was able to load the generated/built Helloworld model directly through Simulink and see the expected results. Further, I can change the model, rebuild/load and see the changes reflected. This leads me to believe that something was hung up and flashing a an ELF file through S32 fixed it.
For science: I did start my test model by saving a copy of one of the example files (to get model properties), such as the UART echo test. Then I changed the Target Config Information block to MPC5775E.