Nevertheless, the question remains: what’s p’shat in this phrase?
One of the major themes of Bereishit (there are 4 or 5, and a thorough study of them all would probably be book-length) is that of migration. Careful attention to the Torah’s usage of the roots gur, shev, chai, and shochen yields a framework for the entire narrative. Ve-acamo”l. Here, two are merged – ger and toshav, normally denoting two different things, are conflated. Why?
At the Brit Bein Ha-Betarim, Avraham was promised that his seed would be ‘ger’ for 400 years. Ramban points out that since the 400 years begins with the birth of Yitzchak, from the time of his birth, Avraham and his descendants are no longer ‘toshavim’ in the land of Canaan, but are gerim. This status of ger only applies to whomever is currently the bearer of Avraham’s brit. Avraham and Yitzchak, at the end of their lives, once they have already designated successors and ‘passed the torch’, are no longer subject to the travails common to the Chosen Family. Yaakov also wishes to enter into that state of ‘retirement’ from being covenant-bearer (see the first verse of Va-yeshev and Rashi there. Note also the interplay between these two roots, shav and ger, in that verse), but that plan backfires, ve-acamo”l. The end of their lives are recorded in ‘fast forward’ mode, similar to Terach’s, their deaths even recorded prematurely. Once they have passed the torch, they are no longer essential to the story.
By the end of the Akeidah, it is certain who will be the heir of Avraham’s Covenant (until that moment, when Yitzchak’s death was expected, there was still some doubt). Avraham has passed the torch. He is ready to ‘retire’ (this theme actually comes up in several places in the Parsha – where Avraham’s ‘old-age’ is mentioned), but he can’t because Yitzchak is not ready to assume that mantle, specifically because he is unmarried (and it seems that the covenant includes Partiarch-and-wife, especially since the bracha of ‘pru u-revu’ is part of the Covenant. Only when Yitzchak gets married, and Rivka replaces Sarah, can Avraham fully ‘retire’ (note also that the eved Avraham consistently refers to Yitzcahk as ‘ben adoni’ until he introduces Rivka to him, when he calls him ‘adoni’ – reinforcing Yitzchak’s replacement of Avraham at that moment).
Between the Akeida and Yitzchak’s marriage, Avraham is in limbo – ready to retire, but unable to do so. This ‘limbo’ is reflected in his self-description as a ‘ger ve-toshav’.