Thank you, RbnJrg, for pointing all this out to me
I can see I've some work to do. I went through SVG explanations at tutorials.jenkov.com.
I followed your advice to adjust the "ftn.html" and the id="ftn1" (or so I believe).
The back-reference works in SIGIL both ways in that it points to the fotnote at "ftn.html" and back to "main.xhtml" from the footnote file.
But it doesn't work both ways in the kindle ereader. While it corrently points to the footnote, it doesn't point back to the original xlink:href location at "main.xhtml".
I couldn't find the solution at tutorials.jenkov or I may have missed something.
I'd really appreciate just one more finishing touch to this as I'm unable to sort it out.