After much reading, I realized there are relatively few "documented" cases of people successfully formatting a black and white interior with Scribus, on a Linux and without Adobe Acrobat.
It's all very confusing. I wrote to IngramSpark, asking whether they would accept PDF 1.3 for a B/W interior, but I am afraid the response will not make things any clearer.
If I don't want to use any proprietary software, it seems I'm stuck between three equally uncertain options:
A) ignoring the PDF/X-1a requirement and instead submitting PDF 1.3 with output intended for grayscale. I suppose this would give good black print quality.
B) ignoring the grayscale requirement and going for a PDF/X-1a with US Web Coated SWOP v2. While making sure that all the file contains no other color than black. Two problems with this: the file might be rejected for using the ICC; the use of an ICC color profile might potentially make the printed text less black.
C) a combination of A and B: grayscaling the images (either with "image effects", or with Gimp) and activating color management. This way, the images would be presumably without a color profile, even if the output intent would have one.
I guess it's time for me to give up, lol.