The merits of a baal teshuva vs. those of an FFB, in parshat Toledot

I recently saw a post at BeyondBT which brought me to the following dvar Torah, excerpted below:

Rashi explains "lenochach" by saying that Isaac and Rebecca prayed in opposite corners of the room. We can imagine how they prayed for hours, intently, for a child, Abraham in one corner and Rebecca in the other, until G-d answered their prayer. Why then does it say: "the L-rd let himself be entreated by him [Isaac]"? What was wrong with Rebecca's prayers? Rashi explains this again, by saying: "By him, and not by her, since the prayers of a tzaddik (righteous person) who is the son of a rasha (wicked person) cannot be compared to the prayers of a tzaddik who is the son of a tzaddik." (Isaac's father, Abraham, was clearly a tzaddik, and Rebecca's father, Bethuel, is considered here to be a rasha.) This seems a little shocking, and unfair to Rebecca. Why should her prayers be ignored because of her father?

Furthermore, this Rashi seems to be contradicted by a well-known Gemara, which says that in the place of a baal teshuva (someone who "returns" to Judaism), not even a completely righteous person can stand. The argument for this is that it is easy enough to be observant if one grows up in an observant environment, but a baal teshuva, who tries to become observant later in life, is usually forced to make changes in his or her lifestyle and circle of friends, which can be quite difficult. How can this be reconciled with Rashi's statement?

Besides whatever answer is offered, I think I can provide a fairly straightforward answer. When the gemara uses the terms "complete tzaddik" and "one who repents" (baal teshuva), this is not the same as FFB and BT, terms which are in use nowadays. The modern baal teshuva and the baal teshuva of the gemara are distinct.

For example, we can readily say that Rivkah was a complete tzaddekes, even though she was not Frum From Birth. As the midrash testifies, based on pesukim, she was a betula entirely even though the people in her place had found ways around the mere technical requirements. And the water arose for her, in her merit. And she showed remarkable gemilat chessed when facing the camel test.

Indeed, Rivkah was never a wicked person. A baal teshuva according to the gemara's definition is one who once sinned, and has now repented. It is not someone who was brought up in a non-religious household. And therefore she can be a tzaddik gamur and yet not a tzaddeket bat tzaddik.

And Yitzchak also was never a wicked person. And so he was not a baal teshuva. Therefore, considerations about baal teshuva vs. tzaddik gamur are irrelevant. And so the gemaras are non-contradictory.

To understand tzaddik son of tzaddik and tzaddik son of a rasha, a good starting point would be Yechezkel 18. But there is much after this. Perhaps an idea of zechus avos; perhaps something else. But regardless, midrashically there was this distinction between Rivkah and Yitzchak in this respect.