Can you revert to iPhoto with 10.10.4 with its library on an external drive?

Download the free trial of iPhoto Library Manager and test, if you can run it, if you do not have a compatible iPhoto version running on your Mac. iPhoto Library Manager works in large parts by scripting iPhoto so you may have problems to run it, since you do not have iPhoto installed. I cant test this because I do not want to uninstall iPhoto.

So what OSX are you currently running Leonie?

MacOS X 10.10.4 on my MacBook Pro and iMac I am currently working on, and a "MacOS X-That-Must-Not-Be-Named" on my Macs for experimenting.

iPhoto 9.6.1 is working very well on MacOS X 10.10.4, as is Aperture.

The upgrade to iPhoto 9.6.1 has not been a problem, at the time it has been released, right before Apple stopped selling iPhoto with the release of Yosemite. It just unfortunate, that you missed the window of opportunity, when upgrading was easy. because the AppStore is so restrictive with available updates, it is usually easy to update to the most current versions, but we need to give the AppStore Support a call, if we need versions that are no longer sold.

Guess I'll have to contact Apple Support to see if they'll provide 9.6.1 and if they do, will go back to Yosemite...

Ask the Mac App Store Support for Help, not the general Apple Support. The App Store has a "Contact Us" link on the main Featured tab in the Quick Links section. The App Store Support should be able to give advice, even if you no longer have Apple Care. What you need is a redeem code to be able to download the 9.6.1 update. Try to convince the App Store Support, that this is an emergency, because you otherwise could not update to Yosemite. You will need to provide a proof of purchase for your current iPhoto version.

So couldn't find anyone with iPhoto 9.6.1 but was able to get a redemption code from Apple Support after calling them a second time...

Thats's great! 🙂

To redeem your code look at the main page of the Mac App Store, the "Featured" tab.

To the right is a panel with "Quick Linls", and one of them is "Redeem".

Sign into the App Store from the "Store" menu, then click the "Redeem" link.

"The photo library needs to be upgraded to work with this version of iPhoto. Your photo library will not be readable by previous versions of iPhoto after the upgrade. The upgrade process for very large libraries may take an hour or more to complete."

That is the routine caution when you open an iPhoto Library created by an older version of iPhoto.

I strongly recommend to make a backup copy of your iPhoto Library on a backup drive, just in case. That will make it possible to repeat the upgrade of the library if something unexpected should happen, like iPhoto crashing or similar.

If I continue, will this basically migrate the previous version iPhoto library to this version?

The library will be upgraded - it is essentially a rebuild of the library to a new format, and only the new iPhoto version will be able to open this library.

iPhoto may need additionally storage during the upgrade. Make sure, that you have plenty of free storage before you start this.