But is not the protection of human life one of the most important--if not the most important--responsibilities of government?
Not at all, the theory of social contract and, as a derivative, government, dictates that it is the responsibility of the tribe/state/etc. to protect its members
, in our instance citizens...often this actually requires the destruction of human life in the form of war with competing tribes/states. In this particular situation citizenship comes at birth and, accordingly, so does governmental responsibility for protection.
If the unborn human life is just as fully human as the postnatal human life, then does not government have the obligation to establish laws to protect that life?
That's a big IF, with which I disagree...but as I pointed out above, the answer to your question is no.
Secondly, if human life bears the image of its Creator, is it right for us to define when an unborn human life form becomes human beyond merely following the tradition of the Church--this is best seen in our great feasts of the Annunciation to the Theotokos and the conceptions of the Theotokos and of St. John the Baptist--that human life begins at conception?
Yes, it is good that we consider this issue for ourselves, these patristic theories were developed with an inaccurate understanding of the reproductive processes. Had these fathers actually understood the details of the phenomena they were commenting on there is no guarantee that their opinions would have been the same. We should not be blindly confined by ancient customs derived from an ignorance of the biological process; rather, we should realize that we are now in a position to evaluate this issue, superior to any other position seen in the history of the Church. Thus, we should consider this issue through the lense of an accurate understanding of the phenomenon, independent of past prejudices based on ignorance and misunderstanding.
While there are many issues on which we can find wisdom in the fathers, there are other issues on which they are simply not qualified to comment. This is through no fault of their own, for they were intelligent and learned men, but a direct result of living in a less advanced time and, thus, not having access to the knowledge and understandings we now enjoy, through no virtue of our own.