Recover Photos from user partition - unrooted HTC M8 MM 6 All photos have been deleted from my Wife's un-rooted HTC M8 running MM. How can I recover the photos?

Ok some background:
HTC One M8 running Marshmallow stock received via OTA update
Photos deleted from Settings\Storage --
Then the phone received OTA MM 6 update
Noticed all photos gone.
Due to MM One click &amp; kingo root no longer work

The phone is not rooted so recovery options like Dr Fone see nothing.
1) How do I back up the phone before rooting?
Backup under MM6 seems to need Google online backup only. It says wait 24 hours but nothing seems to have happened... 
This seems like a chicken or egg issue how do I perform a Nandroid backup without Root to get a full image? To have a chance of restoring?

2) The Root instructions over here need you to have TWRP

3) OK to install TWRP to start the systemless root for MM.... I need to unlock the Bootloader.... which by all reports will loose all data....... this sounds bad when I have no back up.... to get a backup via TWRP I need to load TWRP without root. I've tried the instructions here but I get an error after running the cmd
fastboot flash recovery twrp.img

I get this error

(bootloader) signature checking...
FAILED (remote signature verify fail)

Suggestions from old pre MM posts suggest trying 
fastboot oem unlock 
.... didn't work

4) Can I install TWRP on an unrooted M8 without unlocking the bootloader and therefore loosing everything?

5) Will loading the TWRP image file loose all data anyway?

6) Can I root an M8 without wiping the phone? Do I have to unlock the bootimage to install TWRP? the Systemless root seems to suggest otherwise but to get TWRP on the phone need the bootloader unlocked .... catch 22?

7) Can I use commands from SDK (without root) to take an image of the partition from the phone.

8) Am I basically screwed with no chance of recovery?
No Root means I need to wipe the phone to unlock the bootloader to load TWRP to make a backup of a wiped phone... unless??
If I managed to install TWRP &amp; Root the phone after the MM update (after photo delete) will the partition be so badly overwritten there's no chance of recovering the 2GB of photos?

Sorry for the essay.

Thanks in advance for any help you can offer.
To answer your questions in order:

4) not currently.

5) no, TWRP itself does not lose your data. Flashing on locked bootloaders does.

6) not currently. Pt. 2) You do not have to unlock bootloader to install TWRP. You can also flash it via su dd after you root.

7) impossible, at least with SDK. If you know a JTAG specialist or an FBI tech person who owes you a favor, they might be able to recover your photos.

8) no, you are not screwed. There are a few recommendations below, and no, rooting won't "badly overwrite" your data.

Because I know how important photos are, I would suggest:

Option A: Since your photos are already deleted, you might as well unlock the bootloader and root. Unlocking and rooting shouldn't physically overwrite the sectors of the /data partition. (If they were important, life-changing photos I wouldn't have the balls to do this IMO.)

Option B: Buy your wife a new phone (same phone, different phone) and put the M8 in a drawer for about a year. By then, there should be one-click-root apk's for MM, or something better than that, where you won't need to wipe the device again.

Additional reading: Why unlocking bootloaders wipe the device - Android Enthusiasts