Return to stock issue - can't unroot

Hello,

My tmobile HTC One M8 was rooted (Supersu), unlocked bootloader, with TWRP. For the purposes of upgrading to Marshmallow, I restored stock recovery, re-locked bootloader, but was not successful in uninstalling SuperSu using Full unroot. The process hangs on 'Uninstalling'. I tried factory resetting from stock recovery, but after restarting the device SuperSU is still installed and rooted. Trying Full unroot again failed. Root checker confirmed that the phone was still rooted.
I tried loading the nandroid backup here , and installing from TWRP, but it fails to load ('Error flashing zip '/sdcard/......zip').

Is there another way to unroot the phone? Thanks much.

(bootloader) version: 0.5
(bootloader) version-bootloader: 3.19.0.0000
(bootloader) version-baseband: 1.23.213311491.05G
(bootloader) version-cpld: None
(bootloader) version-microp: None
(bootloader) version-main: 4.20.531.5
(bootloader) version-misc: PVT SHIP S-ON
(bootloader) serialno: ---
(bootloader) imei: ---
(bootloader) imei2: Not Support
(bootloader) meid: 00000000000000
(bootloader) product: m8_ul
(bootloader) platform: hTCBmsm8974
(bootloader) modelid: 0P6B13000
(bootloader) cidnum: T-MOB010
(bootloader) battery-status: good
(bootloader) battery-voltage: 0mV
(bootloader) partition-layout: Generic
(bootloader) security: on
(bootloader) build-mode: SHIP
(bootloader) boot-mode: FASTBOOT
(bootloader) commitno-bootloader: 6768a7b1
(bootloader) hbootpreupdate: 11
(bootloader) gencheckpt: 0