Kernel Panic OS X El Capitan: Unable to get past login screen + Amazon Reward

Hi, I've gone the official route w/ Apple and I'm tired of repeating myself
and before I bring my MacBook in to a repair shop, I'm hoping someone
can guide me to fixing this. I'm not a developer or a savant but I can take instructions.

Will happily send a gift certificate from Amazon to whoever can help me w/ this.

Couple things:

* I caused this. Idiotically, I double clicked on an OS update pkg/installer for Sierra, I believe, while cleaning out unused apps. "Dude."
* I had a recent Time Machine backup until I tried to access it on an external. (Seagate hoses me again.) Can't get anything off the 3Tb.
Friend says its toast. I believe him. Unreal. If I could punch a company, it would be Seagate. Pox on me for giving them another chance to save a buck.
* I basically get to the login screen, get the left-aligned list of PANIC, KILLs and KERNELs and then it's off to the merry-go-round of failed boots.
* I can access Terminal, Startup, etc, but it blocks me (Mac Hard drive = building database -- for 3 days now)...other options are "locked" (external + phantom drive that appeared after my mistake)
* From all I read, I need to do a clean reinstall of Mountain Lion (mine = 1.4.11)

* I can see my files and folders  but they're all greyed out in finder windows. How do I access and save files?
* How can I install OS X when I can't access Store or a "locked" hard drive? 

Oh, and it's my only machine. I have iPad and iPhone.

Thanks for reading. Kind of urgent. Amazon gift card in play.