FYI Amazon could lock you out at anytime for tethering, as it's against the terms of service you agreed to.
Also, OTG is a pain to deal with. Just because the wires are so sturdy that it just topples the Kindle over, the original anyway. I would not recommend it. Since the HD has Bluetooth, you are better off using a keyboard that is also Bluetooth.
As for storage, none of the options are really great. You COULD do it, and people are doing that on the rooted/different ROM Kindles, but again, it's pretty ugly.
The main problem is that you have to use a powered USB hub for most things, so you end up having the Kindle + the OTG cable + the hub + whatever you're hooking up. So you have wires all over the place. I have all the stuff to plug in my little keyboard, but I never use it because it's just not worth the hassle.