Getting someone else's reward

Avot 5:3 is a sort of sequel to the mishnah preceding it (discussed on Avot Today here). Both describe God as being slow to anger, waiting for ten generations before responding to the continuing decline in human behaviour:

There were ten generations from Noah to Abraham, to let it be known how slow God is to anger—because all these generations increasingly angered Him until Abraham came and received the reward of them all.

This mishnah raises many questions, of which one of the most obvious is that of How could Abraham -- or anyone else, for that matter -- receive someone else’s reward?

If we accept that God is good and just and that He would not withhold a reward from anyone who has earned it, we should be able to assume that everyone who lived between the time of the Flood and the death of Abraham did indeed receive a reward or recompense from God for their good deeds, and that Abraham did not receive anything to which he was not personally entitled. Can we accept this is is so and still explain Abraham’s apparently undeserved good fortune in receiving "the reward of them all"? Here are some possible answers:

• Abraham did so many good deeds that he accomplished what it would have been appropriate for all ten generations to have done. It was on this basis that they were all saved in his merit, since he took upon himself the yoke of all the mitzvot in this World. That is why he received a commensurate reward in the World to Come (per Rabbi Ovadyah MiBartenura).
• Since Abraham taught members of his generation to serve God and to keep away from bad deeds, he is associated with their reward “as if he had received it,” and he also received the reward that was appropriate for his own deeds (per Rabbi Menachem Meiri). This explanation distinguishes Abraham from Noah, who was neither a teacher nor a role model: his good qualities did not spread beyond his wife Na’amah and his sons Shem and Japheth.
• The reward Abraham received was one which anyone in any of the earlier generations could have secured for themselves—the reward of being named as the leading Forefather, the Patriarch of what was to become God’s Chosen People. Shem/Malchitzedek nearly secured the same reward several generations before Abraham, but lost the opportunity after he gave a blessing to Abraham before blessing God (Nedarim 32b).
• The names “Noah” and “Abraham” do not refer to Noah and Abraham but are shorthand terms for the generations in which they lived. Thus when we learn that “Abraham” received the rewards of “them all,” we can take it that the generation of Abraham—in which several righteous people lived in addition to Abraham himself—received the aggregate of all the generational bonus rewards that had yet to be conferred (I have yet to find any authoritative source for this explanation).

I suspect that many readers have thought about this themselves and may have reached their own conclusions as to what this mishnah means. Anyone who wishes to share their thoughts on this issue is very welcome to do so.