I was in the exact same spot as you. To answer your questions:
1. No. It will flash over the 4.03s just fine.
2. I would definitely recommend a backup, but you do not need to restore any data(as long as you are coming from stock 4.02, radios don't matter) after the upgrade.
3. Using the ROM I listed this is not necessary(It is completely stock with root).
4. Yes. I have retained root and unlock after flashing. No issues.
Here's what worked for me:
1. I used Wug's GNex Root Toolkit to permanently install CMW Touch(Although you can probably just temporary boot it if you don't want to install) than ran it and made a backup.
2. I went back into android and deactivated google wallet through it's settings.
3. Download the files you want from this
page. I used:
Rooted,De-odexed,Zip-aligned ROM With Stock Insecure Boot Image And New Radios & Bootloader
Enhanced Power Menu Without Native Tethering
4. Back to CMW and wipe dalvik cache.
5. Still in CMW, install zip files.
6. Reboot and reactivate wallet with the same pin and you're good to go.
I have been very happy with 4.04 and the new radios. The keyboard lag has finally been fixed and a few other things as well. Hope it works out.