Confusion! Lets step back a bit…
If you look at the MacBook Pro’s from 2009 onward from the outside you’ll note they all look the same! All are fashioned and shaped almost exactly the same with only minor details changed. Internally the uppercase’s are also very similar. Here the PCB mounts are often located in slightly different locations and then depending on the market the system being sold, the the keyboard will have different keyboard layout and/or keys.
So let’s dive in a bit deeper here the 2011 and 2012 15” MacBook Pro’s use the exact same uppercase design for the logic board. So then it’s only the keyboard & uppercase that is different. First there is two versions of key layout the US version (Horizontal Enter key) and the ISO international versions (vertical enter key) here is the UK version Unibody MacBook Pro 15" Keyboard, UK 09-12 and here is the German version Unibody MacBook Pro 15" Keyboard, German 09-12
So now you see this is a bit of a combination lock!
Finding the right set of parts that will work in your system depending on the series, then the discreet uppercase (US or ISO) Unibody MacBook Pro 15" Top Case Chassis, Int (11/12) and then the given keyboard (UK Vs German). The complete assembly for UK is available Unibody Macbook Pro 15" Top Case & Keyboard, UK (2012) but the German one is no longer available from ‘‘‘TheBookYard’’’ which is one of the better parts suppliers.
As far as the still newer Retina 15” models A1398 that’s a very different kettle of fish! None of these parts I’ve listed will fit as the system was a complete redesign!
Just like your family tree the family name carries forward in this case MacBook Pro A1286 which covers all of the 2009 to 2012 models have this same family name.
But, clearly your uncles children are genetically different that you. So if you needed a kidney their kidney would not likely match yours and would be rejected!
But, a kidney from your brother or sister who'll be a better or even an exact match to yours!
Just as I went over above there are some common elements that over the history of the family line carry forward (you mostly have Blue eyes). But, expecting a discreet part from the oldest models to transplant to yours is no different than getting your great, great grand dad's kidney from his crypt and transplanting it into you! Not a good idea.
You need to stick with what I laid out don't over think this! Don't deviate!
Here’s the answers to your additional Q’s:
1 - No!
2 - Review what I gave you for P/N's per the different uppercase & keyboard possibilities. Frankly, I would just get the parts from a trusted source than in a back allies of Amazon or eBay.
3 - Yours Yes! Older No!
4 - Review the iFixit guides otherwise contact Apple for their service manuals, opps! Can't do that! As they won't release them!!
Help us fight for the Right to Repair! As it turns out some have leaked out and can be found ($$). But, to be truthful the iFixit versions are just easer MacBook Pro 15" Unibody Mid 2012 repair guides.
Don’t forget to score the answer and accept it as answered!