Why did He say 'you will drink from the river"?
Radak: This is how you will be sustained.
Malbim: Eliyahu stopped the conduits of influence from Above; Yisrael would need to subsist on what they had. Therefore also Eliyahu could not receive a Brachah in his food and water. He would get only from what exists already; (a) Water from a river destined to dry up, and (b) Food from ravens, which have a cruel nature, so he would remember that he was cruel on Yisrael to kill them via hunger, which is a raven's nature (it does not feed its young).
What is the meaning of "I commanded ha'Orvim"?
Radak #1: I put in the ravens' hearts to bring to you bread and meat twice daily.
Radak #2: "Orvim" are merchants - "v'Orvei Ma'aravech" (Yechezkel 27:27). Hashem commanded that no one know where Eliyahu is! Perhaps the merchants hid him.
Did they bring to Eliyahu only Kosher meat?
Radak: Yes, since they brought to him amidst Hashem's will. There were still observant people in Yisrael [from which to take Kosher meat] - "v'Hish'arti v'Yisrael Shiv'as Alafim
