1-can we say?
he drives the most dangerously. When comparing between more than two
Yes, we can- it sounds a bit strange without the context, but it's fine.
2- why do we add TO in the first sentence while we do not in the second ?
I do not need TO study. VS. I need not study.
In the first, 'need'is an oridnary verb, but it can also be modal verb (in the negative), which it is in the second.