Contacts on Mac-Mini disappeared after turning on iCloud

I ditched the flip phone 3 weeks ago and got an iPhone SE. Used the Yahoo mail shortcut on the SE to set up iOS Mail (IMAP) and it worked OK. But, I wanted to be able to choose from different "reply-to" e-mail addresses when sending in iOS Mail, so I deleted the Yahoo mail account from the SE today and manually added the yahoo account in iOS Mail which then allowed me to add my various "reply-to" addresses to the account (for some reason that option was not available when using the Yahoo shortcut to set up the IMAP account).

But, that now meant that I was no longer synced on the SE to the Yahoo mail web-based Address Book that I've built up over the years. No worries, though - I decided to use iCloud to access/sync Contacts (and only Contacts) on my Mac-Mini.

The problem: When I signed into iCloud, on the Mini first, then the iPhone SE, nothing happened as far as I could tell. There was no "syncing" happening as far as I could tell. I created a test contact on the SE to see if it would come over to the Mini, and it was around that time I realized all 1,000 or so contacts on the Mini were gone, except for one or two under my name!!! And at some point I noticed the test contact synced on over to Contacts on the Mini!! So somehow, I deleted the Mini's contacts and then synced successfully with the iPhone SE??

Additional info: I think when I first turned on iCloud on the Mini I may have accidentally turned it off answering a question about it needing Location Services on for Find My Mac. I think I then turned iCloud on the SE and when nothing happened, I went back on the Mini and turned iCloud on again.

So, are the Mini's Contacts lost forever (the contact data is all text, no images or anything complicated)? I did a full backup of the Mini five weeks ago before installing El Capitan though I don't know how or where Apple stores Contacts info). How were they so easily deleted in the first place from the Mini? Is it operator error on getting iCloud to work or could it be a bug of some sort? Is it working because that one test contact migrated over? I know there were iCloud problems when El Capitan first came out last year.

Thank you for your help in advance!!