Many thanks, Uwe, somehow I missed this question (and answer) when looked for the answer.
It's not exactly what I needed but I could easily adapt that code into my witchery
Well... not perfect yet. :/
I made the script to eliminate those section starts which were unneccessary in the merged doc because the page numbers appeared to become continuous. I tried to merge some sample docs: some of them formed a continuous page number interval (for example pages 1-9 in 3 different docs), and there were some that missed 1 or more pages, so there must not be (let's say) page no. 10 in the merged one: page 11 should start a new section after page 9.
In this case page 11 was indeed on a new section but section started on the left side.
I think it can't be a serious problem but I must work on it.
Anyway: the original problem doesn't exist anymore, and that's the main point
There is more to it than this but if it suits your workflow use spreads instead of pages.