Why did Rivkah see fit to turn her request into a command?
Oznayim la'Torah: She began with a request - "Shema b'Koli" - so that Yaakov should receive the blessings; but when she saw that Yaakov, who was in tears at having to cheat his father - Midrash Rabah, 1 was reluctant to follow her advice, she turned it into a command - which he could not refuse - in order to save Yitzchak from perverting justice by switching the roles of the Tzadik and the Rasha.
Ha'amek Davar: Do not think that it is mere counsel, and you may decline!
Bear in mind that Yaakov's Midah was 'Emes.'
How could Rivkah tell Yaakov to trick his father?
Refer to 27:6:151:1.
Ohr ha'Chayim: (a) He has a Mitzvah to honor his mother, and (b) she was a prophet, about whom it says "Elav Tishma'un" (Devarim 18:15), even if the Navi says to transgresses a Mitzvah temporarily (Hora'as Sha'ah).
Malbim (to 27:5): Yitzchak intended that the elite of Klal Yisrael will be from Yaakov, and Esav's descendents will work to support them (like Yisrael support Levi). She recognized Esav's nature, that he would not support Yaakov. Rather, he will be like moths, which consume and corrode everything. Rather, it is better that Yaakov receive wealth in this world and rule over Esav.
Why did she need to say "b'Koli"?
Ha'amek Davar: This implies contemplation in the words to be meticulous about them. She hinted that she speaks with Ru'ach ha'Kodesh, and knows that the matter will succeed.