Area D
Without the SKP itself I can't be sure - can you zip it and attach it to a post [or to a PM if it's confidential].
The Exporter dialog looks OK.
I think it's perhaps a 'modeling' rather than an export/rendering issue.
I suspect that your mesh has some faces that aren't consistently oriented or perhaps overlaid with second faces ?***
Are materials applied to faces or instances ? Faces are recommended.
Can you change the SKP's view mode to Monochrome using a Style where front and back face colors are distinctly different - say buff fronts and bright-blue or magenta backs.
Then you'll see if any faces are the wrong way round.
To fix these View > Hidden Geometry 'on' and edit the container [if any] and select reversed faces and right-click context-menu 'Reverse' to flip them to match adjacent ones. If it's a one mesh sheet [or manifold solid etc] you can select a correctly oriented face and use the context-menu 'Orient' command instead - then all connected faces will flip match it - don't do this if there are complexly abutting faces as the result will be a mess!
Please report back, and if the issue persists let's see the problem SKP too...
***PS: After some messing on I have also managed to reproduce something like this - and get black-patch artifacts on a 'smoothed' pillow in an imported 3rd party bed SKP. It's NOT reversed faces as these would show 'buff'...
There was also a similar small artifact on the bottom end of the bed-cover...
However, I found that a simple re-smoothing of that pillow component etc removed these artifacts when it was re-rendered, so all was OK.
I have made a compilation of the two - an original 'faulty' and a 're-smoothed' version: there are SKP and OCS screen-shots of them below...
So, my recommendations are try re-smoothing any problem group/component with coplanar-smoothing and smoothed at ~70deg...
The Monochrome tip given above is still useful... BUT I suspect that 'over-smoothing' is perhaps at the heart of the issue...
Works for me...