Verify which keyboard layout you have chosen in the upper right corner, this is not a driver issue...
My keyboard gets detected as a Canadian one. I have tried switching to an American one using the upper-right corner (among other layouts) too. But still I can't type the slash key or use shift.
However, maybe I inserted the .ffs files incorrectly. The input is still not perfect. What I did is I just inserted Usbkbdxe in the free space after all the Ozmosis modules. I inserted the Apple module after that one. I apologize if this is a bit of a newbie question, but was I supposed to do something different? For example, does Usbkbdxe replace an existing module in the firmware?
UPDATE:
It never occurred to me to use my recovery key to get in. So I used that and the login screen was stable enough to accept all the characters in the recovery key. Once logged in, I changed my password. So if anybody is trying this out, make sure you set a local recovery key and write it down. You'll need to be prepared that the password you choose might not work on the login screen.
Thank you everybody for your help and patience, especially considering I don't even speak your language!