iphoto 9.6.1 events all mixed up

Now many of the events no longer contain the photos that should be there.

That's a corrupted database. A preferences file has no role in tracking what decisions you've made in your organisation and editing.

Download iPhoto Library Manager and use its rebuild function. (In early versions of Library Manager it's the File -> Rebuild command. In later versions it's under the Library menu.)

This will create an entirely new library. It will then copy (or try to) your photos and all the associated metadata and versions to this new Library, and arrange it as close as it can to what you had in the damaged Library. It does this based on information it finds in the iPhoto sharing mechanism - but that means that things not shared won't be there, so no slideshows, books or calendars, for instance - but it should get all your events, albums and keywords, faces and places back.

Because this process creates an entirely new library and leaves your old one untouched, it is non-destructive, and if you're not happy with the results you can simply return to your old one.

That said, working with iPhoto is the deadest of dead ends. The next machine you buy won't even run it. But you don't have to use Photos, there are plenty of alternatives depending on your budget and needs. What I would urge you to do is make your choice now and do the transition now. Doing it now, while iPhoto still runs, will be a pain where the sun doesn't shine. If you wait until you're forced into it, by an upgrade or purchasing a new Mac, then it's going to be a lot tougher to make that change.

Then do the following:

1 - make sure you have a full and current backup of your libraryl

2 - launch the library with the Option+Command keys held down and select "Rebuild Database" from the options.

If that is successful I'd also repeat with "Repair Permissions".

Your MacBook Pro can run Big Sur and I recommend you do so and convert your library to Photos. Photos will convert each Event

into an album with the same name as the Event. Those albums will be in a folder in the sidebar titled (drum roll please…): iPhoto Events. If you click on the folder you'll see all of the Event albums in the right hand pane and they can be sorted by Name or by Date either oldest first or newest first or manually:

It is so much more powerful than iPhoto with the option to edit photos from within the library with any image editor in your Applications folder. The only caveat is that the edited photo must be saved in the same format and the original, i.e. .jpeg to .jpeg. NOT .jpeg to .psd (Photoshop).

What happened before your events got mixed up? Have you moved the iPhoto Library to a new location or have you been running any cleaning applications to remove duplicates? Putting the library in an unsupported location like a Dropbox or on iCloud Drive or a NAS can damage it badly, also running aggressive duplicate detectors.

Events are organized chronologically, but they may be displayed in different ways. In the event view try to set View->Events->by date

What year and model ID is your MBP? Also how much RAM do you have and how big is your boot drive and how much free space do you currently have on it?: