LVM mirror with failed disk can't be repaired

I'm trying to help someone resurrect a RAID1 mirrored LVM logical volume on an old server running Fedora 20 (lvm2-2.0.2.106, linux 3.19.8-100). One of the disks in the mirror is dead. The disk belonged to a VG named systemvg. vgchange says:

vgchange -a y --partial systemvg
  PARTIAL MODE. Incomplete logical volumes will be processed.
  PV qtva6m-iBhc-q3Zt-At1U-FiWH-iZd8-tjpR0u not recognised. Is the device missing?
  device-mapper: reload ioctl on  failed: Input/output error
  1 logical volume(s) in volume group "systemvg" now active

(The "ioctl on failed" is puzzling, but whatever.)

Now we try to repair the LV named "home" in that VG:

lvconvert --repair systemvg/home
  PV qtva6m-iBhc-q3Zt-At1U-FiWH-iZd8-tjpR0u not recognised. Is the device missing?
Attempt to replace failed RAID images (requires full device resync)? [y/n]: y
  device-mapper: create ioctl on systemvg-home_rmeta_2 failed: Device or resource busy
  Failed to suspend systemvg/home before committing changes
  Failed to replace faulty devices in systemvg/home.
  Device '/dev/sdc' has been left open (1 remaining references).
  Device '/dev/sda1' has been left open (1 remaining references).
  Internal error: 2 device(s) were left open and have been closed.

The situation is now complicated by the fact that we (foolishly?) added a new PV (new blank disk) to systemvg using vgextend, in the hope that this would help. It didn't, and now we can't remove that disk from systemvg.

lsblk currently shows:

sda                                  8:0    0  7.3T  0 disk 
└─sda1                               8:1    0  7.3T  0 part 
  ├─systemvg-home_rmeta_2          253:9    0    4M  0 lvm  
  └─systemvg-home_rimage_2         253:10   0  5.5T  0 lvm  
sdc                                  8:32   0  5.5T  0 disk 
├─systemvg-home_rmeta_0            253:2    0    4M  0 lvm  
└─systemvg-home_rimage_0           253:3    0  5.5T  0 lvm  
systemvg-home_rmeta_1-missing_0_0  253:4    0    4M  0 lvm  
└─systemvg-home_rmeta_1            253:5    0    4M  0 lvm  
systemvg-home_rimage_1-missing_0_0 253:6    0  5.5T  0 lvm  
└─systemvg-home_rimage_1           253:7    0  5.5T  0 lvm  

sdc is the remaining original half of the mirror. sda is the new empty disk we added to systemvg.

/dev/mapper contains:

systemvg-home_rimage_0
systemvg-home_rimage_1
systemvg-home_rimage_1-missing_0_0
systemvg-home_rimage_2
systemvg-home_rmeta_0
systemvg-home_rmeta_1
systemvg-home_rmeta_1-missing_0_0
systemvg-home_rmeta_2

but no "/dev/mapper/systemvg-home".

How can we recover the data from the remaining half of the mirror?