I have to admit that I really cannot play on the extension past Eb unless I am seated.
As for the humps on the backs they are nice because sometimes you do need to play something like a low D that is followed by C# or something to that affect. Something like the opening in the 4th movement of Beethoven 5 or letter N in Beethoven 7 1st movement (I have been told to play the cello line sometimes). Sometimes you could a bit of fancy latch work, but sometimes I find it easier and safer to just use a finger.
As far as fast passages I know exactly what you mean by sharp edges being dangerous. Though for me it is only those on the tuners that I worry about as I play most fast extension work without putting my thumb behind, kind of like a really low and reversed thumb position hand.
What is most important I think though it get what works for you, and is comfortable (or safe perhaps
).