My goal: Return the phone to completely stock condition. No root, locked bootloader and preferably S-ON. I've read a few stories where HTC insisted on replacing the motherboard (with an additional full price fee) because the device was S-OFF and I don't want to go there.
Background: I used firewater to S-off the phone as soon as I got it (it was running 1.12 at the time). I have flashed the Harmon Kardon audio stuff (http://forum.xda-developers.com/show....php?t=2737498). I have turned off the tamper flag (http://forum.xda-developers.com/show....php?t=2708565). I am running the Stock+ rom which is based on 1.57 (http://forum.xda-developers.com/show....php?t=2716843).
1) Reset my bootloader to locked (http://forum.xda-developers.com/show....php?t=2708571)
2) Wipe the phone and restore my very first nandroid backup I made with TWRP. That will get me back to unrooted 1.12 firmware but without the stock bootloader and still S-OFF. Staying with 1.12 will allow me to S-OFF again with firewater if it comes to that. If some kind of emergency came up, I'd hate to spend $25 on sunshine for a one time use on a phone that is going back to HTC forever.
3) Install the stock bootloader that matches 1.12 firmware. Not sure how to do this yet. I'm afraid I won't be able to achieve this with a locked bootloader and no root.
4) Verify everything is back to stock.
5) Set the phone back to S-ON (http://forum.xda-developers.com/show....php?t=2758818)
6) Run the 1.57 RUU (http://forum.xda-developers.com/show....php?t=2714456)
7) Proceed with the warranty exchange.
I'm not sure running the RUU is 100% necessary, but I figure it's the best bet I have to make the phone completely stock.
Questions: Is this a sane plan? Am I doing anything in the wrong order? Are there any steps that won't work because I've lost root or unlocked bootloader? Am I setting myself up for a brick anywhere?
There is a thread about doing this on a EU M8 (http://forum.xda-developers.com/show....php?t=2735235) but I can't tell what, if any, modifications need to be done for the USA T-Mobile version of the M8. There is talk about having to flash a firmware file twice or risk a hard brick when setting S-ON. Do I need to be worried about any of that?