I was not aware of these narrations. They seem interesting. Can you provide the name of the book where I can look into these narrations.
Normally we Shias believe Imams to be Hujjat-ULLAH. I don't have any narration stating Prophets to be Hujjat-ULLAH but I myself believe Prophets also to be Hujjat-ULLAH as Nabi Ibrahim (a.s) was Hujjat-ULLAH in his time.
As I stated before I am not knowledgeable in this field and everyday I learn something new.
I don't believe in those narrations but at least the earlier Imami view made more sense than the Imamis of today who claim that WE MUST HAVE A leader and guide BUT the man's absent so... sorry. Then they blame it on us that he's absent Haha.
As for the Prophets and Messengers, they ARE the strongest of God's many arguments:
{[We sent] messengers as bringers of good tidings and warners so that mankind will have no argument(Hujjah) against Allah after the messengers. And ever is Allah Exalted in Might and Wise.} [4:165]The two narrations are authentic according to your scholars, both from al-Kafi.
There's other authentic ones from basa'ir al-Darajat for instance:
يعقوب السراج قال
قلت لأبي عبد الله عليه السلام تخلوا الأرض من عالم منكم حي ظاهر تفزع إليه الناس في حلالهم وحرامهم فقال يا أبا يوسف لا
I said to abu `Abdillah (as): “Would the earth be devoid from an apparent man of knowledge from among you [O household], so that the people may seek him in their issues of Halal and Haram?” He (as) replied: “O abu Yusuf, NO."
عن محمد بن مسلم عن أبي جعفر عليه السلام قال لا تبقى الأرض بغير امام ظاهر
from abu Ja`far (as) that he said: “The earth shall not remain without an apparent Imam.”