Since nobody else has mentioned it... did you try the pin reset? Find the small hole in the back of the Kobo, insert pin to press button it should reset for you. A simple search on how to do a reset would have at least told you that much.
It should also be noted the Kobo DOES NOT RECOMMEND sideloading the firmware. This is what happens when people don't read. Of course this type of thing is probably why Kobo will never have an open beta setup. Too many fools who don't know what they are doing and then crying foul.
Oh that note you CAN get back to 2.1.5 by sideloading the APPROPRIATE firmware for YOUR Kobo. You WILL NOT be able to sync your Kobo with the desktop app afterword however as it will try to update your Kobo again. There is a thread here somewhere (I can't find it at the moment) where you can edit a file, change it's version and the Kobo won't try to update it's firmware.