How much is a Kikar? Why does the Torah reckon 1775 Shekalim as a partial Kikar?
Rashi (on Pasuk 24) and Rashbam: 1 Kikar actually equals 60 Manah, and a Manah = 25 Sela'im (Shekalim), 1 in which case 1 Kikar = 1500 Shekalim. Only since a Manah shel Kodesh is double that of a Manah shel Chol, 1 Kikar = 3000 Shekalim.
The Shekalim mentioned in the Torah are all Sela'im. Throughout Shas, 1 Sela = two Shekalim (See Targum Onkelos and Targum Yonasan on Pasuk 26).
Why does the Torah say "Kesef Pekudei ha'Edah", and not Kesef ha'Terumah or ha'Tenufah, like it says about the gold and copper?
Da'as Zekenim: The Torah gives the amount of silver that were obligatory (a half-Shekel for each man). It does not say how much they gave voluntarily. Hadar Zekenim - the voluntary silver went to a storehouse for Bedek ha'Bayis and needs of the Tzibur; Moshav Zekenim - and for Korbanos Kitz ha'Mizbe'ach.