What is the significance of "Leil Shimurim Hu la'Shem"?
Ramban #2, Hadar Zekenim: He was waiting for the moment that they would be worthy of redemption.
With reference to the four special nights that are recorded in Hashem's Book of Chronicles: 1. The night on which He revealed Himself to create the world; 2. The night on which He revealed Himself to Avraham (at the Bris Bein ha'Besarim); 3. The night on which He revealed Himself in Egypt, when with His Hand He killed the Egyptian firstborn, whilst with His right, He saved those of Yisrael and 4. The night on which He will reveal Himself to redeem Yisrael from among the nations (See also Targum Yerushalmi).
Since this was the night that He fixed at the Bris Bein ha'Besarim, and He was awaiting its arrival (Ramban).
And what is the significance of "Leil Shimurim l'Chol Bnei Yisrael"?
Rashi: And it is the night that Yisrael is protected against destructive angels. 1
Ramban: The night that Hashem waited to take Yisrael out of Egypt is (sanctified) for Hashem for all generations, for them to serve Him by eating the Pesach and remembering the miracles (Hagadah)and by reciting Hallel and giving thanks to His Name, 2
Seforno: Just as Hashem waited for the night of the redemption from Egypt, so too, does He wait to redeem them [from the current Galus]. 3
Targum Yonasan: It is the night that is guarded against destructive angels and to redeem them from the nations forever.
Hadar Zekenim: Also Yisrael wait for the night when they can celebrate the laws of Pesach.
Rashi writes that it is the night that Yisrael is protected against Mazikin. The Mechilta says that it is a night that needs guarding!
Moshav Zekenim: Its Mitzvos require guarding - "u'Shmartem Es ha'Matzos", "u'Shmartem Es ha'Davar", v'Shamarta Es ha'Davar ha'Zeh."
Rashi writes that it is the night that Yisrael is protected against Mazikin. If so, why do we [begin to] read the Hagadah in Arame'ic (which angels do not understand Mechilta says that it is a night that needs guarding!
Moshav Zekenim #1: The Mazikin cannot enter [our houses to harm us] by themselves, but if [they hear and understand that] we invite 'anyone who needs, let him come...', they would come.
Moshav Zekenim #2: The reason it is in Arame'ic is not due to fear [from Mazikin].