Why did he swear?
Malbim: The Ge'ulah depends on two times. "B'Itah Achishenu" - if we merit, Hashem will bring Mashi'ach early. If not, he will come in the proper time (Sanhedrin 98a). He could not swear that the Ge'ulah will not be earlier, for they could merit that it be earlier! Rather, he swore that it will not delay more than this time. The afflictions said above will cease then. According to Mahari, Yishmael's rule will cease then. Since there was a Shevu'ah, it cannot be retracted in any case 1 .
An oath can be permitted via regret! Rabah bar bar Chanah heard a voice from Heaven say 'who will permit My oath?!' (Bava Basra 74a). (PF)
Why did he swear in "Chei ha'Olam"?
Ibn Ezra: One swears in something that persists, like "Nishbati v'Kodshi" (Heichal ha'Kodesh) or "Esa El Shamayim 1 Yadi." Chei ha'Olam is Hashem, who gives life to all.
The Rambam (Hilchos Shevu'os 12:3) rules that a Shevu'ah in Shamayim va'Aretz does not take effect! (PF)
When is "l'Mo'ed Mo'adim va'Chetzi"?
Refer to 7:25:4:1-3 and the notes there.
What is the meaning of "uch'Chalos Napetz Yad Am Kodesh"?
Rashi: When Yisrael's strength will cease – "Ki Azlas Yad v'Efes Atzur v'Azuv."
Malbim: When the time for Am Kodesh to be scattered finishes.
Malbim citing Mahari: When finishes the rule of the Sarim standing on the river bank. Then Malchus Shamayim will begin, like I explained "v'Lei Yehiv Shiltan vi'Ykar" (refer to 7:14:1:2).
