Is it possible to temporarily keep your iPhone from draining the macbook battery? When I'm out and about I sometimes want to connect my [nearly empty] macbook to my [not so empty] iphone so I can tether without using WiFi (saving battery), but then my poor macbook starts charging my iPhone and is drained even faster.
Not possible, you would have to physically alter the USB cord and remove the pins related to power and leave the data ones intact.
That's not possible. Also, modifying a USB cable (as suggested in another answer) is a) probably not a good idea and b) probably won't even work (see this question & answer). Energy-wise, you are most likely better of by just using wifi for tethering.