You have analyzed the situation very well, I don’t have much to add.
As explained some time ago in this thread, for RGB input images (TIFF or JPEG), PhotoLab uses the input color space as working color space, which explains your observations.
Different programs use different approaches, some of them use a fixed working color space and will convert the image to that at the beginning, some let you choose. PhotoLab currently does not provide such options. You could still do such a conversion manually before processing your images in PhotoLab, for example using ColorSync.
For displaying the preview, we convert the image to the display color space at the end of our processing, that’s why the test image appears correctly.
For RAW images, on the other hand, we always use AdobeRGB as working space, for all cameras.
If you think that the option to chose a working color space different from the input color space would be useful for TIFF and JPEG images, please file a feature request.
Br, Wolf