Vayetzei - Unity Amid Disparity

Why does Yaakov place 12 separate rocks under his head before he goes to sleep, and what is the concept behind them combining to form a single rock? Why were Avraham and Yitzchak, as well as Adam Harishon, not able to have 12 tribes? What does the midrash mean when it says that the high priest enters the holy of holies with the merit of the twelve tribes? What do the twelve stones of the breastplate of the High Priest have to do with the twelve stones of Yaakov? Why does Yaakov keep mentioning the word 'this' when he wakes up from the dream? What do these stones have to do with the foundation stone which is on the Temple Mount 'til this very day?