@guiverc great idea making this whole conversation public as it will mean others might be able to jump in and offer advise.
At some point, I need to implement our whole email system on Discourse so new topics can be emailed here. That way you can easily echo any message to the mailing list at the same time since there are people there that don’t pay attention to Discourse (and vice versa) but that’s not really relevant to this particular topic.
So I’m not sure you saw the further conversation on IRC but tl;dr Calamares’ philosophy about manual partitioning is that you proceed at your own risk. It won’t prevent you from doing something impossible (meaning here, outside the capabilities of a filesystem or partition table scheme). I think Occam’s Razor would suggest this to be the parsimonious explanation, especially in light of the complex partition scheme and the fact that manually partitioning outside of the installer didn’t seem to be any more successful.
One thing that might be helpful to help us visualize and work through this is screenshots from Calamares of what the initial scheme is like as well as what you’re trying to change it to. Perhaps that might provide some insight.
BTW I agree this is not high priority at all.