As the OP, I want to try to clear up some of the confusion. I have caused plenty of it. :-)
I drop-kicked the partitions I had set up and started over with a bare (nothing but free space) sda block device. At some point I have expressed doubt that my PC could handle UEFI correctly. This is not true. I have been able to install multiple Debian systems (jessie, stretch, and sid) using GPT and LVM. These same systems have also been installed on this PC using the old primary/logical (dos) scheme with LVM, starting with a bare disk. This desktop PC, according to dmesg is: HP Pro 3400 Series MT/2ABF, BIOS 7.16 03/23/2012 Placing faith in the installer and letting it do as much of the work as it will seems to be a good approach. YMMV. Also be unafraid to power-off and re-boot the installer if you can't figure a way out of some difficulty. Hey, after a few dozen of these, you may even start to relax! The backup (/home and /var) has definitely been tested. IIRC, the installer handles a greater proportion of the job in the primary/logical LVM scheme and manual partitioning. In contrast, with GPT and LVM, for the second and subsequent installations, the partitioning and defining of PVs, VGs, and LVs needs to be done before installation using gdisk and the LVM tools in /sbin. Then, the installer with manual partitioning can succeed by defining the mount points of the various LVs. - Dan
