Q: Lost almost all my photos after uploading to iCloud!!!

I couldn't get into my photo library at all so I eventually and stupidly clicked the "Delete Incomplete Items". This took my photo library from 17500+ photos to 900.

Do exactly as the message tells you.  After you clicked the delete button,  Photos has removed all photos, that have no longer been on your Mac anyway - only thumbnails with the original photos missing.

Now set this library as your system photo library in the "Photos > Preferences > general". Then open "Photos > Preferences > iCloud" and enable iCloud Photo Library.  Wait for your photos to download back to your Mac. That may take several ays , if your library is large.

The question remains, why your library all of a sudden was no longer the system library and iCloud has been disabled.

Where is your Photos library? In your Pictures folder or on an external drive?

Did you move the library to a new location, or did you sign out of iCloud?  What changed before the problem first occurred?

If the photos don't download, you can also try going to iCloud.com and then select iPhotos. At the top right where your name is, click the dropdown menu, and select iCloud Settings. When that page loads, look at the bottom left side under Advanced and you can restore from there.

The problem is that after clicking the 'Delete Incomplete Items' button it's also removed all these photos from iCloud itself. I had previously viewed the 17500+ photo on the iCloud web interface but now I can only see the 900 remaining photos. My iCloud Storage has also dropped significantly. I assume that means all hopes of restoring these photos from iCloud are lost?

How can Apple think that this crappy dialog is enough explanation to delete the majority of my photos off their server! This was supposed to be a safe place for my photos to make sure I didn't lose them and it's done the exact opposite.

Fortunately I have found a backup of my Library in Dropbox and it looks like I've only lost a few months (much better than a few years). I'm going to get this backup onto an external HDD before going any further.

Once I've done that I still need to figure out how to fix this issue with iCloud. How do I go about using this backup of my Photo Library to re-upload the missing photos without making an even bigger mess?

Have to say I'm pretty disappointed with this service so far.

The problem is that after clicking the 'Delete Incomplete Items' button it's also removed all these photos from iCloud itself. I had previously viewed the 17500+ photo on the iCloud web interface but now I can only see the 900 remaining photos. My iCloud Storage has also dropped significantly

That is not is supposed to happen. 'Delete Incomplete Items' should only delete ' Incomplete Items' from the local copy of the Photos library and not the complete items in iCloud. The iCloud version of your library must have had an issue as well. I'd contact Apple Support and see if they can help. It looks like a dangerous bug to me.

Once I've done that I still need to figure out how to fix this issue with iCloud. How do I go about using this backup of my Photo Library to re-upload the missing photos without making an even bigger mess?

iCloud Photo Library is not a backup. It is the central working storage of your photos, if you are using iCloud Photo Library. It is meant to keep all Photos libraries on all devices updated and  in sync.   You cannot use iCloud Photo Library to restore photos you delete from your photos Library.  If you delete a photo from a Photos Library that is syncing with iCloud Photo Library, the photo will be deleted from iCloud as well, and from all other devices syncing with iCloud Photo Library.  If you want a cloud backup, use a different service.