A garage is obligated in a mezuzah as any other storage room, with a berachah, especially if it is attached to the house. Even though nobody “lives” in a storage room, it is fully obligated in a mezuzah if it serves the owner or renter in some way and he enters into it from time to time.
The answer to your second question would depend on the two acceptable opinions cited in our discussion regarding wide doorways. The general custom seems to follow those authorities who rule that the mezuzah should be affixed on the brick or cement walls on the outer tefach that is still under the cement lintel.[A] Consequently, one would measure from the floor to this lintel to determine the top third.
According to the other opinion, the mezuzah should be put on the railing as it would be considered a fitted post within a wide doorway. If one follows that opinion, he would measure from the floor up to the casement and place the mezuzah on the top third of the railing.  Since it is under the lintel, it can be affixed facing out.
To answer your third question, each of the garage doors requires its own mezuzah.
 Shulchan Aruch 286:2; Mezuzot Beitecha 286:14, citing Teshuvos R’ Akiva Eiger 66; Parashah Sedurah 16; Agur B’ohalecha 34:4:9; Az Nidberu 3:58, Aruch HaShulchan 286:9 reports that the custom is to make a berachah. Teshuvos V’Hanhagos 1:647 allows a berachah only if the garage is attached to the house. Cf. Minchas Yitzchak 10:96, who rules that one should not make a berachah on any storage room.
 Agur B’ohalecha 34:9, citing Shach 286:2 and Noda Bi-yehudah O.C. 2:47.
 In this case, the outer tefach of the garage is also the outer tefach of the house.
 Chovas HaDar 8:3; Sha’arei HaMezuzah 12:9; Aruch HaShulchan 287:7; Pitchei Mezuzot 28:10.
 Agur B’ohalecha 22:3.
 Agur B’ohalecha 14:4. Since the mezuzah is within the doorway and under the lintel, it may face outward. This is different than the case of doorposts that only have space on their outer face which is outside the lintel’s cover.