For the buttons, the first step is to figure out what KeyEvents they are generating.
Download
http://www.temblast.com/usbmode.htm
What do the buttons do, VolUp/Dn or PgUp/Dn?
Push a button for half a second. Does the down triangle appear immediately or just a quick down/up when you release the button?
You could use some solution that remaps the buttons in a context sensitive manner. So for one app it's PgUp/Dn, for another it's VolUp/Dn. I think that's a stupid idea.
Me? It's an ereader. I'd have the buttons be PgUp/Dn period. If there is an app that is a problem I'd fix the app. The Onyx ecosystem might not allow easy modding as it uses sharedUserId. But you can take a lone 3rd party app, mod it and resign it without creating too much trouble.
On the EDL front, I would presume that the EDL loader is the same as the Poke3 (SDM636) for the Leaf2 (SDM630).