Default Can't attain nvflash in Nexus 7

I bricked my 2012 Nexus 7 in APX mode. I've ordered a replacement motherboard for a $50 fix. Meanwhile,the replacement 2012 N7 I ordered came in. Naturally, I want to make a bootloader image from this device, so followiing this guide, I unlocked the bootloader, flashed the flatline_grouper.img recovery, and flashed the AndroidRoot BL bootloader. Then I made the blob.bin bootloader image,

I can now get in and out of APX mode in the new N7 using wheelie, but I can't attain nvflash status, because the "wheelie -b -blob.bin" command doesn't get me into the bootloader from APX mode. It says it completed sending the bootloader, and my device should be ready for nvflash, but the Nexus 7 stops at the Google and unlocked lock screen rather than continuing to the bootloader. Nvflash commands other than nvflash -h return "unknown command" messages.

Could the problem be that the N7 came with Android 4.2.1 and its newer bootloader? I flashed back to 4.1.2, prior to changing the recovery and bootloader, since nvflash is set up for the 4.1.3 bootloader.

Anyway, I can't attain nvflash. Has anyone managed to get it? How? What am I doing wrong?