Thanks for collecting this in one place and summarizing your findings.
For Affinity Designer, my work document is in inches and when the document is exported as an SVG there are no options for units in the SVG export menu (as far as I can tell). So apparently the export is always defined as pixels without any ability to change it.
As a test, I was able to create an SVG directly in Inkscape which the GFUI was able to load. I do not use Inkscape regularly so that’s not a real option for me, I just did it as a test.