Answer to: The power pads for the MacBookLook for the power-on pads directly above the keyboard connex cable. Two square pads, horizontally arranged above the keyboard connector (when display hinge is oriented at the top).
Answer to: Is battery from 2015 MBA the same as the MBA 2011No. These batteries are not interchangeable. They are shaped differently (close, but not close enough to work), and they have different logic board connex.
Answer to: Can I use a Mid 2009 screen on a Mid 2012?Odds are that it's just the inverter board that's killed the backlight on the display. Repair involves removing the display module, and removing the LCD panel from that module in order to get access to the inverter board so it's not for the faint of heart -- and of course you'll need a replacement inverter board as well. Not likely to be the LCD panel or the main logic board (MLB) though as you've got video out and pixels on the panel. The panel is likely the same but the cabling lengths and placement of connectors may be different as the internal layout of the MacBook Pro likely changed a good bit between the 2009 and 2012 models. Proceed with care. -- Joe, too.