I did have unimesh enabled for a number of the renders (I labeled which ones in the images I posted.) And yes, enabling unimesh used to fix those issues for me... but now it doesn't. That's the whole problem in a nutshell, really.
The creases are definitely along the body part borders... Not the material borders, but the body part borders. You can see in the attached file where I compare a Poser real-time screencap with the abdomen highlighted, with an Octane area render of the same area. The images aren't perfectly aligned or scaled, but you can clearly see the shape of the abdomen reflected in the creases on the Octane render.
The attached Octane screenshot, by the way, is done with the model in Unimesh and the Octane smoothing angle set to 80.00, so clearly that doesn't fix the problem either. Also, for what it's worth, while I have mostly used a single scene to demonstrate the problems, it shows up with other characters, scenes and lighting conditions.