Downgrade iMovie Library from 10.3.6 to 10.2.5

That's a tough situation. Once you update the library by opening it in a newer iMovie version the update for all practical purposes is irreversible. The updated library cannot be opened with the old iMovie version as you have found.

If you keep a Time Machine backup you might be able to restore your old iMovie library and iMovie app to an earlier point in time preceding the update. Then you should be able to open the library with your older iMovie version. Restore the library and iMovie app to different locations that the current ones so as not to overwrite them.

Regarding you Show Package Contents question, read the thread in the below link and see if anything in there helps you. No guarantees, and you should do your own experimenting after backing everything up.

Opening iMovie Library reveals LOTS of ev… - Apple Community

-- Rich

It appears you will be unable to open the Library so it might be quicker to simply re-edit the video on your other Mac.

If you have no longer got the original SD card clips you should be able to salvage them from the Library with Show Package Contents>Original Media.

I really think you need to sort out why your Mac is overheating and why the iMovie export is not working on it.

Is it the Library that is causing the problem?

Create a new Library, add some clips and try to export it.

It's a long shot but whenever editing apps start misbehaving, deleting the preferences can sometimes help.

Hold down the Option and Command keys as you launch iMovie and in the popup window click Delete Preferences.

Another thought . . . could the problem be that your hard drive is full?

You should never let it get more than 90% full or you are likely to be in trouble.

How much free space have you got?

For future reference you should not let your Library get too full either. You mentioned hundreds of hours of footage . . . long before it gets to 10 hours you should be creating new libraries. You can have as many as you want.

Incidentally if you are thinking of a new computer, the best value is the base model M2 Mac mini at under £600 from Amazon.

Its performance is incredible . . . far better than any Intel Mac.

Of course it isn't a laptop but it is still quite portable as you can carry it around with a keyboard and mouse and plug it into any modern TV to use as a monitor.

I got one a month ago together with a 27" monitor for £110. I'm using a cheap keyboard and mouse that I already had.

It blows my 2017 27" iMac out of the water being at least 3 times faster.

If you are serious about an M2 Mac mini there are 2 things you need to be aware of.

The base model only has 8 GB RAM and a 256 GB SSD.

When the M1 came out there was a concern that the small SSD would have a comparatively limited lifespan which was made even worse by the fact that the small amount of RAM necessitated a lot of RAM swapping when undertaking extremely heavy processing tasks.

These fears were possibly overblown but as I am a belt and braces guy I got around the "problem" by plugging an old 1 TB Samsung T5 SSD into the mini, installing macOS Sonoma on it and running the mini from the OS on the Samsung. (The latest 1 TB Samsung T7 costs £82 from Amazon).

This solution gives me a low cost 1TB drive and any wear and tear occurs on the T5 which is comparatively cheap to replace.

As for performance, I was expecting to see a drastic reduction but all the Benchmarks I did were identical and using Final Cut Pro and DaVinci Resolve felt just as fast as using the internal drive.

If the battery is dodgy can't you run the MBP off the mains electricity?