-Will the Macbook Air charge faster with a Pro adapter?
Don't ever do this, it could permanently damage MBA.
-Could damage result from under-powering my MacBook Pro? (like a brownout)?
Actually, there's a few things interesting with how power system work on MBP.
As far as I know, my Retina MBP runs at 12392 mV (12.392 V) while I'm just browsing, and bumps up a lot when I'm gaming. So, it is possible to give it only 14 V, but it will charge extremely slow, and "Please Don't Try That If You're Ready To Take Risk! More Importantly, Don't Blame Me For Any Damage!"
Also, I want to mention one thing, that is the MagSafe Airline Adapter:
This adapter by Apple connects to Airplanes, and they are basically one end MagSafe, another end this EmPower thing:
It's 15V, and at the end of that Wikipedia page it reads,
Apple offers an EmPower Magsafe power adapter for their MacBook, MacBook Pro, and
MacBook Air lines of notebooks, and also includes a cigarette lighter
socket adapter. However, this system only runs the computer and will
not charge the computer's battery, and Apple indicates that users
should not plug this device into a car's cigarette lighter outlet.
So my guess is, it will work, but not well.
It's ultimately your choice, and if I were you, I'll test it secretly on my friends' laptop.