@sotnik - try the good old KISS (keep ist stupid, simple) - and ask yourself some questions
a) Should siduction have the whole disk?
b) are there other systems that should stay
a) let calamares reformat and partition the whole disk
b) ok, some things are possible
* start a live system and create a new partition for the new system. let calamares replace that partition
* replace one existing partition with the new system
c) manual installation - configure the system right from the begin to fit your needs - and please don't forget things. Sounds easy, but isn't, common pitfalls: UEFi install without a partition for UEFI, Legacy installation with gpt, forget the partition for the start system etc
The main problem i see is that there are some combinations of faults possible that are not fully covered by upstream defined bugs - the most obvious are, but not all. Result: Not working system. In such cases it would be nice to analyze it and file an upstream bug - but i think the very most people prefer a simple install and modify the partition layout afterwards. Good for them, bad for upstream
Some things will change with the new kpmcore 3.3.xy and the crypt and lvm integration - these things will improve the handling of trivial bugs too, hopefully. We will see it - or not.