Question: Q: 15" macbook pro has a slight separation

Hi,

You'd better replace HDD with SSD which is getting reasonably price these days. Also upgrade RAM is a good idea.

Your hard drive has failed so replace it with an SSD as suggested by @kaz-k. I recommend using either the Crucial MX500 SSD (good compromise on price/performance) or an OWC Electra 3G SSD. A 2010 laptop uses a SATA II controller so some SSDs may not be able to properly auto-negotiate the SATA link speed since most current SSDs are SATA III devices (yes they should work without issue, but in practice they do not). The OWC 3G SSD is a SATA II SSD so no compatibility issues. Stay away from the Crucial BX500 series as the SSD is a low end economy budget model that overheats and is as slow as a hard drive.

If the battery is swelling, then remove the battery immediately so that it doesn't cause more damage to the laptop as the swelling will only get worse.

The only difference would be for the OWC SSD. A 2012 laptop uses a SATA III controller so if you go with an OWC Electra 6G or Extreme 6G SSD (the 6G indicates SATA III for 6Gb/s speed). Personally I would go for the Crucial MX500 SSD as it is a good compromise on price & performance, but the OWC SSD will work just fine although I haven't really used the more recent OWC SSD which has changed a bit since the last ones I used years ago.

As for memory you should only use memory from either Crucial or OWC since Macs are extremely picky about the memory they use. Make sure to use the tools on their respective websites to get the exact part numbers that are compatible with your Mac. When I purchased memory from Crucial last year they still put "Mac Compatible" on the packaging the memory shipped in. I'm not sure it is even possible to purchase an 8GB stick of RAM for the mid-2012 model at this time since everywhere legitimate I looked for Crucial or OWC memory was sold out (even Crucial themselves). It was still possible to purchase 4GB modules though (I prefer a single 8GB module paired with one of the original 2GB modules to give an inexpensive upgrade to 10GB total).

A 2012 laptop is capable of running up to macOS 10.15 Catalina, but Catalina does not support 32 bit apps so make sure your third party apps are compatible with Catalina.

https://support.apple.com/en-us/HT208436

You need to provide more information here. It appears you installed a Crucial SSD, but now cannot log into your laptop?

How did you create a bootable the macOS installation on the new SSD? Did you install macOS, did you migrate from a backup, or did you clone the original drive (using what process)?

Have you tried using an external USB keyboard in case one or more of the keys on your built-in keyboard are bad?

Try booting into Safe Mode to see if you can log in.

What happens when you attempt to enter your password?

Were you ever able to log into the new SSD? Or is this still your first attempt to log in?

How is your user account configured? Is it set up to use a regular user account password or did you tie your AppleID to the login of this laptop? I have never tied my AppleID to my macOS user account login so I'm not sure how that works.

I would definitely try Safe Mode since that is something very simple to try in case some startup or login item is having a problem.

You may want to perform a clean install of macOS on the SSD by first erasing the whole physical SSD before selecting the "Install macOS" option. Make sure to create a new admin user account (make it something different from the user accounts on your backup) so that you know the clean install boots and logs in properly.

Once you know the SSD is booting Ok, then use Migration Assistant to migrate your original user account. Leave everything else unchecked so you only transfer your main macOS user account to see if you can log into it. You may need to boot into Safe Mode in order to disable any Login items that may be missing (I'm not sure how macOS handles Login items that no longer exist). If you can log in, then you can try migrating your apps (make sure to uncheck the user account since it has already been transferred). Of course you do risk having to start over again or performing more troubleshooting if there are issues. It all depends on your needs, time, and desire to figure it out or to have everything restored from a backup instead of manually setting things up from scratch. Sometimes it is hard to decide. I made several attempts migrating my last Mac, before I decided to perform a clean install and manually transfer my data and reinstall apps since I didn't want to transfer my issues to the new Mac.

<< (I'm not sure how macOS handles Login items that no longer exist). >>

if the login item itself exists in the proper directory for login items, it will be installed, otherwise, ignored.

Select the item on the top line (your Physical drive) and Repair Disk.

Select the line under that,typically the MacOS Volume, and choose repair disk. This time it will take around five minutes to complete, but needs to pass without error to proceed.