First NVflash is locked down with a secure boot key (SBK) and that is why you receive the error. I have a project I am working on to try and get our NVflash working.
What happens when you try flashing recovery.img via fastboot? Are you getting errors or does it seem to be working correctly? I tried flashing HBwelch's beta 2 ROM one time and it relocked the bootloader. With it locked fastboot will not flash recovery.img.
As long as you have fastboot your tab should be recoverable. Try this link for Thor recovery and flash in fastboot. Maybe try
before flashing.Code:fastboot erase recovery