Your hitting an Apple imposted brick wall! Apple puts an expiration date within its OS installers (certificate). Here’s a bit more on it If you've got an old macOS install image, it will probably stop working today
Apple did fix their blonder on the newest versions as they expired prematurely, reissuing Mojave back to El Capitan. Sadly Lion and Mountain Lion did not get updated. While we can get around that using a different approach I think you’ve making this harder than it needs to be. Is there a strong reason why you are going this far back?
If you didn’t altered your drive and only noted your system stopped working (you got the blinking question mark in a folder) Then you have in fact a hardware issues not software!
OK, let me explain… The Unibody MacBook Pro’s often need a new HD SATA cable and more so when you upgrade a HDD to a SSD. Here’s a bit more on the details Your Hard Drive Cable Is A Ticking Time Bomb I think the title says it all!
So here’s the needed part MacBook Pro 13" Unibody (Mid 2012) Hard Drive Cable And yes this is a 2012 version of the cable which is the better one. And here’s the guide to put it in MacBook Pro 13" Unibody Late 2011 Hard Drive Cable Replacement
Even still we need to address two things besides replacing the cable, as you read in the link above we need to place a strip of electricians tape on the uppercase where the cable crosses over so you protect it from abrasion from the rough aluminum. The second is not damaging the cable in the process of installing it. Don’t crease it! That damages the thin foil wires within the cable. Instead find an old BIC ballpoint pen to get the ink straw out, we’ll use it as a forming brake. Using it to create the clean arcs where the cable needs to bend around the corners, its radius is as tight as you want the cable to bend.
While I’ve gone over the hardware side of things you still likely have corruptions on the drive. Hopefully you still have a bootable drive. If not, you’ll need to install a fresh OS. There are a few different ways to do this. The first is to leverage internet recovery which if your system has the newer firmware should be able to gain access to it. With your drive installed restart your system and hold the Option, Command (⌘) and R keys to enter into Internet Recovery making sure you have internet access via WiFi or better via Ethernet.
If you can’t get there, see if you have a recovery partition setup following ‘How to reinstall macOS from macOS Recovery’
The next option is to create using a second Mac a bootable OS installer USB Thumb drive following this guide: How to create a bootable macOS Sierra installer drive using the OS installed from here: How to upgrade to macOS Sierra, jump down to Step 4 for the URL link.
I strongly recommend you stick with Sierra on your system and you need the file I linked here as Apple messed up the certificates on the older versions which brings us back to the top issue!
Let us know how it goes!