Replaced my buddies iMac hard drive (EMC 2389) and upon reassembly his LCD did not light up. Booted up fine. Chimed. 4th...

I had the same issue with my 2010 Aveo. I took it to the shop and got a transmission fluid swap and it seemed to of fixed it for me, maybe low fluid causing some stress.

Yes you can max it at 16gb. You will notice faster load times and performance especially with Lightroom. Im a heavy Illustrator, After Effects, Lightroom user and I maxed my ram out to 16 and couldn't be happier. Unsure if it effects the battery too much. Mine is on the brink of replacement anyways so I can't answer that part clearly. The ifix guide is easy to follow for ram installation and even Apple gives all users a guide as it doesn't void any warranty. Also check with OWC for which ram is required for your system. Mac uses low voltage ram so don't go run to BestBuy and buy any ram. Best to stick to OWC and ifix it guide lines. Cheers