Success!
My system was on NCRS26.58-44-3 having installed the January update from WiFi. The new files you posted were accepted and I was able to lock the bootloader!
One comment:
I followed the entire sequence in OP; no signature errors reported, but both "boot" and "recovery" got the result "(bootloader) Image signed with key bad key".
Then tried to re-lock using "fastboot oem lock". I had this exchange (I removed the recurring "not found" messages):
C:\OWENS_SPRINT_NCRS26.58-44-3>fastboot oem lock
(bootloader) WARNING: This command erases all user data.
(bootloader) Please re-run this command to continue.
OKAY [ 0.203s]
finished. total time: 0.208s
C:\OWENS_SPRINT_NCRS26.58-44-3>fastboot oem lock
(bootloader) Flash valid Android images now
(bootloader) Then re-run this command to lock
OKAY [ 0.206s]
finished. total time: 0.206s
C:OWENS_SPRINT_NCRS26.58-44-3>fastboot oem lock
(bootloader) Still require signed boot.img
OKAY [ 0.188s]
finished. total time: 0.193s
C:OWENS_SPRINT_NCRS26.58-44-3>fastboot flash boot boot.img
target reported max download size of 536870912 bytes
sending 'boot' (16384 KB)...
OKAY [ 0.658s]
writing 'boot'...
OKAY [ 0.697s]
finished. total time: 1.365s
C:\OWENS_SPRINT_NCRS26.58-44-3>fastboot oem lock
(bootloader) Still require signed system.img and/or oem.img
OKAY [ 0.197s]
finished. total time: 0.202s
Given this broad hint from fastboot, I then re-sent the 6 chunks of system and 2 chunks of oem, just to be sure. After that I locked once more and all was well:
C:\OWENS_SPRINT_NCRS26.58-44-3>fastboot oem lock
(bootloader) Lock complete! Flash signed images after reboot if needed
OKAY [ 0.349s]
finished. total time: 0.354s
C:\OWENS_SPRINT_NCRS26.58-44-3>fastboot oem fb_mode_clear
OKAY [ 0.172s]
finished. total time: 0.177s
Full power off and on again, the system booted without "bad key"!
I strongly suspect that the flashing of everything (possibly except boot) initially was unnecessary. The optimal sequence is most likely to start with "fastboot oem fb_mode_set" and "fastboot oem lock" (the latter twice), then flash all the pieces, then the final "fastboot oem lock" and "fastboot oem fb_mode_clear".
I still need to get the device on the network and let in install the later updates from Boost, that would confirm completely that it's back to stock and lock (I suspect there is still a way to see that it had been unlocked).
Thanks again bcrichster!