Can I add non-case-sensitive APFS volumes to a case-sensitive APFS container?

The short answer is no, you cannot add non-case-sensitive APFS volumes to a case-sensitive APFS container. Once a container is created with a certain case sensitivity, every volume within that container inherits that same case sensitivity. So if the container is case-sensitive, all volumes inside it will be case-sensitive as well, regardless of what you selected when creating those volumes.

Unfortunately, the only way to switch to non-case-sensitive is to reformat the entire container, which means backing everything up, deleting the container, and recreating it with the right case setting. That’s a bit of a pain, especially on a RAID 5 setup.

I would check the info pane of your containers and volumes with Disk Utility.

It's my experience that the case-sensitivity on an APFS formatted drive - ssd specifically - is dictated by the volume and not the container.

I can partition a drive and define it as APFS (Case-sensitive) and the info for the device and the resulting container will indicate "Is case-sensitive: NO", but the single APFS volume IS case-sensitive. I can add volumes to the container having either yes or no case-sensitivity.

So as I said, it appears to me that the volume dictates sensitivity, not the container.

I'll qualify this by adding I've never used a RAID backup, so perhaps there are rules there that override the basics of APFS.