For trading, it should be straight forward. You would have to pay capital gains only on your profit. That might be taxed at some concessional rate or may even be exempt from taxes, depending on your jurisdiction.
That's correct. But note that you must be able to provide evidence that you bought BTC at certain price etc, otherwise they can expect you to pay tax based on total value.
In case of any doubts, don't be afraid to ask relevant tax office, as there may still be some uncertainties in interpretation of tax rules.