1)
Why did they need to build a Mizbe'ach? Even if "Beis Kel" is not Shilo (refer to 20:27:1:1), Yakov's Mizbe'ach was still in Beis Kel!
1.
Radak: They built a new Mizbe'ach to seek Hashem.
2)
Why did they offer Olos and Shelamim?
1.
Malbim: It was to find favor in Hashem's eyes.
