We can never be certain about the correctness of this transmission. The same holds true for Shiite principles. This really implies that hadith is the only source of Seerah. I have no problems in accepting this fact.
All histories are, in fact, based on the narration of people.
To further understand the issue, consider the example that you have cited. In such a case, these points may be accepted on the basis of Hadith, unless there is something contrary to the Qur'an, the Sunnah or to any of the established and acknowledged historical or scientific facts entailed in these narratives. The point is not that Hadith is an unreliable source of information. In other words, it is only when compared to the Qur'an and the Sunnah that Hadith seems wanting in reliability. Nevertheless, if one were to compare Hadith with, for instance, sources of other histories, it would easily surpass the reliability of the sources of other histories on all accounts.