Yes, of course if handedness is genetic, i dont know about that and a neurological factor seems more likely, left handedness doesnt have to have an advantage to exist in a population just so long as it isnt a disadvantage in reproducing. It would be just another variation in the human species. This of course is a fundamental principle of evolution, variation must exist to allow evolution to move in any particular direction if need be.
There are humans with plenty of movement in their toes but i think this is generally a good thing for those wishing to swing through trees ie. monkeys!!
Humans are humans cos it was an advantage to lose that ability and replace it with feet better for walking and running than gripping tree branches.
Yes, evolution allows variation to occur so long as it is not detrimental to the organisms ability to reproduce. If this wasnt the case natural selection would have nothing to work on.