Re: Frame shape
So get another nut; it's just not a big deal. Make sure it's good and tight. Don't pedal the thing without the cranks firmly fastened, or you can wallow out the square hole in the crank arm.
As for your earlier comment about using the stronger brake on the rear, remember that due to forward weight shift when braking, the rear wheel skids MUCH more easily than the front. You can get about three times as much braking on the front wheel before your bike will begin to tip up, than you can get on the rear wheel before it skids.
Just about every time I have seen someone go over the bars from braking with the front, it was not from excessive braking force. It was from them flopping forward into the bars like a ragdoll and turning the bike over in the process. When braking hard, just push back against the bars as hard as necessary to hold yourself in place, and you won't do that.
"When I see an adult on a bicycle, I do not despair for the future of the human race."