HmdToEyePose is the same value-pair provided in ovrEyeRenderDesc. Modifying this value is suggested only if the app is forcing monoscopic rendering and requires that all layers including quad layers show up in a monoscopic fashion.
HmdSpaceToWorldScaleInMeters is used to scale player motion into in-application units. In other words, it is how big an in-application unit is in the player's physical meters. For example, if the application uses inches as its units then HmdSpaceToWorldScaleInMeters would be 0.0254. Note that if you are scaling the player in size, this must also scale. So if your application units are inches, but you're shrinking the player to half their normal size, then HmdSpaceToWorldScaleInMeters would be 0.0254*2.0.