Having a 2×1 key (a la backspace) in the top right corner would broaden the selection of keycap sets you can draw from. OTOH, if you're planning to only make one keyboard for yourself, this isn't a big deal.
Why do both of you put your fn-layer arrow keys on OKL; ? Wouldn't IJKL make it more accessible?
(To be honest, I've considered for a while just training myself to type from a position where the index fingers lie on FK, not FJ, and then an OKL; arrow cluster would be perfect. I don't suppose you've had the same thing in mind?)
Actually having the two keys in place of the backspace doesn't require any special keycaps at all. You could easily just take any of the Esc, F1-F12, Ps/SR, ScLk, Pa/Br, Ins, Home, Pg Up. All of those keys would fit where the backspace was. (They are sculpted keys, the top 2 rows use the same profile, and the bottom two rows use the same profile. Thus why the arrow keys are all shaped the same. In my case I could purchase a 87 key set and have all the necessary keycaps to create my keyboard. Although TheProfosist would still need a custom size shift key on the right, but other than that they could do the same buying a 87 key set.
The reason for placing the arrow keys all the way to the right is because if you place your hand on your keyboard with your pinky on the menu key and then place your other fingers on P L ; ' you will notice that your pinky doesn't have any strain. If you count the keys you will notice neither of us used "O K L ;" and "I J K L" is too far to the left for your pinky to be comfortable. If you need to use your second hand for hitting the arrow keys then that is why I put a second set of arrow keys where "W A S D" is.
On a side note I do not type with index fingers on f and j, I used to be heavy into gaming and so I trained myself to basically type 70% with my left hand so my right hand would be free to use the mouse for faster controls. I also type 90 wpm still even with a strange hand position like that. I hit Y with my left hand, and I hit B with my left hand. I hit H with either my left hand or right hand. I wouldn't recommend learning to do this unless you plan on being a gamer in a future life, I just learned it young and couldn't correct it, sounds like I adapted well though right? lol