Two Years Later - An Answer
Over two years ago, I asked this question on something that bothered me in the parsha.
This morning, I came across an explanation in Chok L'Yisroel (citing Tanchuma 2 and Pirkei d'Rabbi Eliezer 38) on Rashi's comment that finally helped me understand. It stated:
Hashem agreed to go along with the brothers' agreement for three reasons:
1) Had Yaakov known that Yosef was really alive, Yaakov would have bought back Yosef and not have gone down to sojourn in Egypt, and consequently, the Divine will of (Bereishis 15:13), "Know for sure, that your offspring will be strangers in a land that it is not theirs" would not have been fulfilled;
2) Yaakov might have cursed Yosef's brothers for what they had done, and they would not have numerous offspring, whereas Hashem wished them to multiply; and
3) Hashem wished to separate Yaakov from his son Yosef for 22 years, just as Yaakov himself had avoidably stayed away from his own father, Yitzchak, for exactly that period of time.
UPDATE: See the latest posting on Heichal HaNegina which addresses the third reason listed above.