ProjectionCenterOffset, which is in GetStereoProjectionMatrix, actually shifts around the resulting projections, changing where the images converges for you. If you look at the math in there, it’s a translation that’s applied to the final projection, so it basically shifts the projection around on the projected plane. Usually, this is used for lining up with optics on the HMD, but you may need to use it too, depending on your setup, in order to make the image converge. However, if you want to change the amount of parallax, adjusting the IPD in CalculateStereoViewOffset() is the way to go.
For Yoshi, the module should be set up as a module. Unreal doesn’t associate one module with one class, so you’re free to have as many as you need in one module. If you need a template, the SimpleHMD plugin is a great place to start. You can copy that, and then just replace the FSimpleHMD class with your class, which is based off of FFakeStereoRenderingDevice. The difference is that you’ll just be making a stereo device, and not an HMD, so make sure you inherit from IStereoRendering, instead of IHeadMountedDisplay. You’ll also need to set up the engine to use it. I’d suggest doing this in StartupModule() for your module, and just directly set GEngine->StereoRenderingDevice to a new instance of your class.