For what it's worth, I can reproduce the bug. I'll be as thorough as possible; if any further information is desired, let me know.
Setup:
- the system is a normal (CTR) 3DS, US region
- sysNAND was 9.2.0-20U, downgraded to 4.2.0-9U
- emuNAND is the same as old sysNAND: 9.2.0-20U
- GW / Launcher.dat is 3.0.1
- Cubic Ninja retail cartridge has ninjhax 1.1b installed.
Procedure:
- Power on 3DS, run DS Profile exploit and hold L
- Boot into Classic Mode
- Insert Cubic Ninja cart; load game; enable Wifi
- Press A -> Create -> QR Code
- ninjhax begins to load;
- the 3DS hangs at the 70% mark.
A few extras, just for fun.
- SysNAND, no GW involved, booting retail cart: selecting QR Code crashes the game, giving an error message and a forced restart. Predictable, considering 9.2.0-20 is not the same as 4.2.0-9 (or whatever this apparent chimeric monster really is).
- EmuNAND, booting Cubic Ninja ROM, scanning ninjhax QR code for OLD 9.2.0-20U: "installs" to save file just fine, running the exploit reaches 70% and then hangs.
- EmuNAND, booting ROM, scanning ninjhax QR code for OLD 4.2.0-9U: the 3DS hangs. No visible activity at all.
What concerns me the most is the impact this will have on version numbers.
"What's your firmware?" "It's 4.5.0-10U." is suddenly ambiguous.
We'll end up requiring nightmarish constructs like "It was 9.2.0-20U, DG'd via GW 3.0.1 to 4.2.0-9U, UD'd to 4.5.0U via AC:NL."
To speak nothing of people who try to buy a 3DS with "firmware <= 4.5"...
Oh well. Thank goodness for NAND backups. I don't have a hardware mod, but I guess it's time to get one.
EDIT: Just tried another scenario: untouched mode, load CN cart, L+R+X+Y to delete save, scan QR code for OLD 4.2.0-9U; ninjhax loads, installs, boot.3dsx (the homebrew launcher) runs, and I can run yeti3DS. At least for me, ninjhax is fully working with a retail cart, 4.2.0-9U chimera sysNAND, and OLD 4.2.0-9U ninjhax 1.1b QR code.
EDIT2: I realized that, in this case, I can restore sysNAND to 9.2.0-20U without a hardware mod. So, that's a good thing. Assuming it works.