I figure I'd ask since you didn't mention it in your opening post.
There is another topic from someone running 2.20.0 who has no issues installing the firmware. So since 2.3.0 to 2.20.0 work, I would think 2.7 should also work.
Could you specify which regional version of the access point/firmware you have? I know there are issues with flashing the firmware on Canadian models, so perhaps yours also has additional checks that aren't in the "universal" firmware.
To my knowledge, nobody's been able to figure out the root password, or another way to obtain a root shell on these devices.