Why did he say "Ki Cho Amar Hashem"?
Malbim: Now, it was not within Eliyahu's power to bring Brachah (because he stopped the conduits of influence from Above). It came only because Hashem said so. This is unlike Elisha's miracle of the oil (Melachim II, Perek 4), which was due to his power to bring Brachah. The latter can only increase what already exists; Elisha first asked what she has in the house. Here, a Brachah due to Hashem's decree, is on the Kli "Kad ha'Kemach Lo Sichlah v'Tzapachas ha'Shemen Lo Sechsar", even if it is initially empty.
Why is it written Titen, with a Nun at the end, and we pronounce it Tes?
Radak (here and Shmuel II, 15:21): The Kesiv is like "Lesiten Sham" (6:19). Either way it means the same. In the first exile, Seforim were lost, and Chachamim who knew the Mikra died, and Anshei Keneses ha'Gedolah [who codified Tanach] found disagreement and followed the majority. When they did not reach clarity, they wrote one [version] without Nekudos (it is written, but not pronounced), or they wrote one outside, but not inside (it is pronounced, but not written), or they wrote one outside and one inside (it is pronounced unlike it is written).
