Reinstall Original OS?

I made the mistake of buying a new MacBook Pro M1. It came with Big Sur. I saw that my 2014 MBP was 'compatible' with the old MBP and I 'up-graded' it to Big Sur for continuity. My old MBP gets very hot and the battery discharges much faster with Big Sur; I measured 111 degrees at the center of the keyboard and it was getting hotter. It was much hotter, by far, then it had ever run.

I contacted Apple Support about reverting back to MacOS 10.15, which ran very well on that device. Big Sur is not easy to circumvent. I was told I had to go back to the original MacOS, which was MacOS 10.10, and it is not easily available. I could not find it. While talking with Apple Support, they emailed MacOS 10.10 to me and I began the long process of upgrading to 10.10. All along the conversation, one moment the OS would be termed numerically and the next moment they'd be calling it a geographical name. Apple don't do that, I know you guys are required to know all those details but I'm just a user, OK, a Customer.

The upgrade to MacOS 10.10 was chugging along and I could not stay up all night so I went to bed, letting the computer run. During the night I guess the computer crashed, or froze, or maybe got lonely and stopped. The next morning, I had to try waking it up and it would not go. I called Apple Support and arranged to take the MBP into the Apple Store to have geniuses look at it. He said he'd note to have them install, incrementally, MAcOS 10.10; 10.11; 10.12; 10.13; 1014 and 10.15, so to have a fully functional computer.

I took the MBP into the Apple Store and they said that they'd install it up to MacOS 10.15 for me but I'd have to leave it over night. I left it with them and I got a notification that it was ready to be picked up the next morning. I picked it up and took it home. once home I turn it on and found it had been updated to only MacOS 10.14. I should have left it there, but there was something incompatible with several of my hard drives. I attempted to up upgrade one more step unto my desired MacOS 10.15 and hit a snag. I dialog implied a problem with a partition. I have never had a partition on that MBP and didn't really understand why there would be one on there, but I clicked OK and things went crazy from there. Yes, I somehow ended up with three partitions, all with 512GB of data, on a 512 GB drive!!!!

Although the computer seemed to function, I had a hard time finding applications and documents at times. After several days, I was working on backing up data to a Hard Drive and there was the sound you hear when you send an email. I checked the eMail Program and all my eMails from 2017 through 2020 had disappeared. I had most of them filed by month and year and even the folders they were stored in were gone. This prompted another call to Apple Support. They suggested I delete a partition and I did. I chose the partition that seemed to have very little data on it and the computer crashed. It now crashes half-way through startup.

I have most of this data backed up but I cannot backup a Mac that doesn't boot.

I have to arrange to once again to have the good folks at the Apple Genius Bar reinstall MacOS 10.15. I hate to bother these people.

Ironbarny