Dead MacBook Air?

This is totally frustrating.

I purchased a new MacBook Pro about 18 months ago since my mid-2012 MacBook Air was close to ten years old. At the time, the MacBook Air was running fine, but a bit slow. It also did not qualify for the newest Mac OS so I decided to upgrade. I also backed up the entire hard drive on to a USB drive.

Since then, the old MacBook Air has been sitting next to my desk, plugged-in but never used. As I am reshuffling the furniture from my office, I decided I would use the MacBook Air as a music production computer. I tried turning it on last night and received numerous errors. Also, the battery has apparently died and is stuck at 2% charge. After messing with it for several hours, I decided to wipe the hard drive and reinstall the OS. Well, that took another six hours. The progress bars always lie about how much time is left. After everything is done, the computer is surprisingly slow. I did not feel the computer was sluggish when I used it a year ago, but now, every click or even scroll will take seconds to minutes. Since I deleted and reinstalled everything, there is plenty of room on the hard drive. Right now, I am trying to download and install Chrome browser, but there are constant errors in Safari so I cannot even download any apps.

I do not know what could have changed. Would I be in this predicament or worse if I did not buy the new MacBook Pro? Are these issues caused by the computer sitting unused for over a year? Whatever it is, it is frustrating to keep staring at the rainbow wheel.