Why did Hashem tell Eliyahu to relocate? He could have arranged that Eliyahu get water there, just like He caused him to get food!
Refer to 17:7:1:1.
Malbim (7): Hashem had commanded Eliyahu to go to the border of Eretz Yisrael to stop natural rain, which comes from vapors that rise from rivers and other water sources. Now that the great river, and surely all other water sources dried up 1 , no natural rain would come, so Eliyahu did not need to be at the border.
Below (18:5), Achav told Ovadyah to go to the springs and rivers to seek fodder! Perhaps he meant, even though the springs and rivers dried up, perhaps the ground there is still moist and has fodder. Or, Malbim means that there was so little water in the water sources that it would not produce enough vapors to bring rain. (PF)
What is the meaning of "Tzarfasah"?
Rashi: It is to Tzarfas (this is the name of a city).
Why does it say "Asher l'Tzidon"?
Radak: There is another Tzarfas - "Asher Kena'anim Ad Tzarfas" (Ovadyah 1:20). Eliyahu went to Tzarfas near Tzidon; it is in Eretz Yisrael.
Did Hashem command the widow to give to him? We find that initially she did not want to give to him (verse 12)!
Rashi: I commanded in My entourage that a widow there will feed you.
Radak: Initially, Eliyahu thought that He put in her heart to feed him. When he found that she does not have enough to sustain herself, he realized that Hashem 'commanded' a miracle that will enable her to feed him. This is why he said "Ko Amar Hashem
Why did Eliyahu need that someone else feed him?
Malbim: Brachah could not come on Eliyahu's food (because he stopped the conduits of influence from Above). She was a Tzadekes, and proper for Brachah to descend on her to keep her alive. She fed him via that Brachah.
