The obligation of the first blessing from the rabbinate

In the blessings for her, the Gemara seeks a source for the first blessing.
Rabbi Akiva learned this from a verse dealing with Neta Reba:
“‘[And in the fourth year all its fruit shall be] holy to the Lord ‘ – teaches that a blessing is recited before and after them, hence Rabbi Akiva said: It is forbidden for a person to taste anything before reciting the blessing.”
The issue continues and brings up two more sources for reciting a blessing before eating:
I said it lightly: When he is full – he blesses; when he is hungry – not even more so?!
… It is understood: It is forbidden for a person to enjoy this world without a blessing.”
Here we found three sources that serve as a Torah law: a sermon, a qu’on, and a sabbarah. And yet, according to all the Rishonim, the first blessing is the rabbinan…

1. A sermon can be said to be a reference and not a binding source. But why say so, and how do we know the difference between a sermon and a reference?
2. What are the ‘flaws’ in the KJV and the Sabra that do not make them Torah?
3. Since the law is truly rabbinical, why does the Talmud look for a ‘source’ for rabbinical law? Many regulations were established by the Sages without a source, but according to what seemed necessary to them. What is the point of finding a (fictitious?) source for such a law?