Parshas Bechukosai and Zechus Avos

How does the concept of “zechus avos” work? Is it that (1) the power of the Avos, or the zechuyos they generated, is so great that it effects all future generations, or (2) future generations who choose to identify with the chain of Jewish history reap the benefits of that positive group association? Is it a top-down process or a bottom-up process?
The gemara (Shabbos 55a) presents a machlokes Shmuel and R’ Yochanan whether there still is “zechus avos” that we can call on. Shmuel’s opinion that “tamah zechus avos” is at first glance astounding – what of all the tefillos we recite which invoke that zechus?! Tosfos offers two answers: The R”I (Tosfos d”h Shmuel) explains that in reality there is no machlokes – zechus avos works for tzadikim (R' Yochanan's position) and does not work for resha’im (Shmuel's position). Rabeinu Tam, however, holds their is a real machlokes here and distinguishes between “zechus avos”, which according to Shmuel has ceased, and “bris avos”, which is eternal. The proof is from our parsha – ‘V’zacharti es bris Ya’akov…” (26:42) Even after the tochacha and galus, there is still a bris avos.
Could these two views of Tosfos reflect the two sides of the chakira? According to Rabeinu Tam there is some powerful bris which effects all future descendents of Klal Yisrael and can never be severed. According to the R”I, zechus avos depends on the degree to which one chooses to conform to the historical mission of Klal Yisrael - it is not an automatic consequence of being a descendent of the Avos.