"Making a whole skeleton from a fragment" is what is done when not enough remains are present to fully describe a species. They compare it to remains of a related and well-known form. It isn't unusual for descriptions to be updated as more info is avaible
Egal, most of the better known dinos are known from largely or entirely complete remains.. sometimes dozens or hundreds. Also fossilization is a very exact process. Not everything that dies and get burried becomes a fossil.
Egal, carbon dating is effective WITHIN A CERTAIN DATE RANGE: ~100,000 years. I'm not allowed nearly enough room in this reply to explain why but trying googling it. NO scientist has EVER used carbon dating on dinos. Colleagues would laugh them to death.