Can anyone fix my ongoing mystery?

So, 4 years ago (and I posted here at the time but it wasn’t very useful lol sorry), my MacBook kind of broke. I’ve ended up leaving it bc it was in pretty shit condition anyway (a key is missing, a headphone got stuck so I couldn’t hear without Bluetooth headphones, the storage was full to the brim so I couldn’t update the Mac and when I would try and delete stuff it would made zero difference). I was also holding out hope that maybe it would fix itself or I’d figure it out but idk anymore. It’s now been lying around for 4 years and im bored of it so im asking again.

So basically, I try to login like normal, but the correct password fails like 8 times. Because of this, I restart my MacBook (I thought it was acting up). MacBook restarts to Home Screen: all trace of the master account I was trying to log into is gone, except the background from the account which is showing behind the Home Screen. Turns out my sister had changed her Apple ID password that day (same Apple is for master account) and while this shouldn’t have affected my MacBook, it is the only catalyst for it not working. She changed her password bc she believed her Apple ID got hacked when she was out.

Extra info:

It is a something 2016 MacBook The MacBook is still on high sierra (because I was unable to update it) I kept getting warnings about a keychain issue before it stopped working along with ‘storage full’ and ‘update your MacBook’ A few months before, my MacBook also kept getting Kernal panics when I’d try start it up and then it would correct itself The account isn’t connected to iCloud When going on the guest account the Bar with all the apps is missing It was VERY slow All this also kind of makes me worry about the security on it bc it could even be like a virus or something?

I still kind of want the stuff off of it and it would be nice to use it again or be able to sell it.

Would recovery mode work?