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you might box up the disassembled MB, charger and etc to a friendly 3rd party mac repair shop. Temporary install a replacement battery and see if your computer will charge it.
-if you can find a shop that is still open (the virus)
-you will have to pay a premium
-what chances the battery u buy online is any newer than the one you got in there just not

best of luck
 
Now tried powering it back up and it took a while longer than normal (which is my new normal unfortunately) but then now I am seeing the Language screen as if it is a new laptop.

Anyone knows what’s up with that?
 
I'd say the spinning hard drive is failing, as your seeing lots of random issues. Need to boot into single user or from an external bootable image and run First Aid on the internal drive.

I've a 15" 2011 and it's been hard used, and it too can be problematic needing some TLC to keep it up and running. Recently the Apple Store opened to a blank Window, requiring some specific cache files to be deleted.

FWIW
2011, 2.4GHz 15" MBP on 10.13, more complex to deal with system issues, 10.12 far easier, however 10.13 is easier on the dGPU and generally more optimised. 10.13 you'll need to revert to terminal far more frequently to resolve any specific issue.

Q-6
 
Eventually got through. I unfortunately haven’t been able to log in properly as yet but at the very least I know my data is still on my hard drive. Don’t know why it opened up and led me to a page to reset my (not forgotten) password but it did show my account with name and image attached so at least I know that’s safe. Just a matter of getting a replacement battery and hopefully that would solve this issue.

At least that’s the direction I’m leaning on since it still powers up after the battery is dead and even with the battery disconnected. Doesn’t look like anything logic board related.
 
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