@sunnydev: The USBNet hack ships with pre-generated server keys on the K3, because the CPU is so tiny that it would take a noticeable amount of time to generate them during setup.
(And OpenSSH's ssh-keygen was broken on the K3 when I did my initial implementation [but that just might have been because of an LTO issue]. Anyway, it's not shipped with USBNet.).
(FWIW: The dropbear keys are the same as ebs' original implementation from back in the FW 2.x days, and I generated the OpenSSH keys myself with OpenSSH-6.0_p1 especially for USBNet. On the other hand, on the K4 & Touch, the vanilla dropbear supports *does* generated server keypairs on first boot).
On the K3: They're in /mnt/us/usbnet/etc (2 for dropbear, 3 for OpenSSH).
So, yes, if you switch from dropbear to OpenSSH, the fingerprint *will* change, they use different keys
1. Do it from a proper Linux host, it'll be faster (and, as I said, ssh-keygen isn't shipped with the hack, so that leaves you only with dropbear's keygen).
2. See the files in /mnt/us/usbnet/etc (The server doesn't need the public key, so they're not shipped with USBNet).
3. See #2.