Why did he build the Mizbe'ach from "12 rocks, like the number of Shevatim"?
Radak #1: It is so the merit of the Shevatim should help that Hashem answer him in his prayer.
Radak #2: It is to show that the 12 Shevatim should be connected and cling to Hashem and offer on one Mizbe'ach, unlike now, that two Shevatim offer on Mizbach Hashem, and the others on Mizbach ha'Ba'al. Therefore, he built the Mizbe'ach in front of their eyes from 12 rocks.
Malbim: We should not err to think that he took 12 corresponding to the 12 Mazalos, like those who serve the Mazalos used to use the number 12.
Why does it mention here that Hashem called Yakov 'Yisrael'?
Rashi citing Bereishis Rabah 82:5: When the Shechinah revealed itself to Yakov in Beis Kel, and called him Yisrael, the same day He told him "Goy v'Kahal Goyim Yihyeh Mimecha" (Bereishis 35:11) - in the future, your children will gather like other nations, and build a Mizbe'ach to burn on it at a time when Bamos are forbidden, and I (Hashem) will be pleased.
Radak: Yakov was in Beis Kel when he built a Mizbe'ach, and it was said there "Yisrael Yihyeh Shemecha" to aggrandize the name of his seed, if they will cling to Hashem. Yisrael is a name of authority - "Ki Sarisa Im Elohim v'Im Anashim" (Bereishis 32:29). In this place where Yakov built a Mizbe'ach, and he was told "Yisrael Yihyeh Shemecha", they made a calf and built a Mizbe'ach to Ba'al!
Malbim: This shows that Yakov [and his descendants] will be above the Mazalos and rule even over angels
