I'm going to be on an aircraft for a long time, but the aircraft offers an ethernet cable, which I can connect to my Macbook through a USB to ethernet cable, however, I would like to get internet access on my iPhone 5S. Is there any way to do this? Like through a lightning cable? Is there a cydia tweak for this? (I'm jailbroken)
Sharing wifi to your phone is a piece of cake. Your computer just can't bridge a network. Go to System Preferences -> Sharing and enable the network. Then connect to it from your iPhone.
Traveling on an airplane would not affect the performance of your computer's wifi, and yes, at least in the U.S., you are allowed to do that above 10,000' feet (although I've never seen them enforce it below).
The FAA has changed rules within the past years that require devices to be on airplane mode (no antennas activated) while below 10,000' but that isn't necessary above. Many airlines actually provide wifi onboard (like many Southwest and some Delta aircraft).
Just spit balling here but perhaps an ad hoc wireless network from your laptop and connect phone to it?