Thank you @[deleted] and @Nationalsolo for your great efforts and help.
I'm currently getting the results I want with v7.10 even when exporting at 30fps. This is why I'll stick with that version now. I'm pretty certain now, that from v7.10 to v7.11 there was an ill-advised change in the order that speed change and frame rate are applied during the rendering process. (Hmmm, this is in disagreement with the fact, that borate managed smooth slowmo with v7.30... Anyway, I have a working solution now and am tired of testing around.)
Once, I feel the need to upgrade, I'll go with what Nationalsolo suggested. However, that would still be a workaround. It may leave only 144fps from my original 240fps. And it would require an additional rendering step in order to bring down the frame rate from 144fps to 30fps for a reasonable file size of the final video.
Thanks again.
LeFouu