I feel your pain. I too would have never imagined that I could make a mistake like that haha. I had a bunch of windows open including an original Kindle root and accidently used it instead of the 8.9 HD version ugh. Also that VCC was definitely the hardest connection. I even left a taped off lead just in case.
As for the Boot.img, I looked around and you are right all of the boot.img files are too big for this type of recovery. I'm certain I got this from the first aid kit and it appears it was different than the files I've seen in the links in the other threads. To help you out why not try the one that I used. It is 4,285kb so should definitely fit. Let me know if it works for you!