Syncing two devices

Hi all. I have a work iPhone and my wife just gave me her old iPad, and I'd like to sync the two. The Apple ID that I had on my work phone was my work email, but I don't want that email associated with my iPad, so I went to https://appleid.apple.com/ and changed the email address.

Now, I'm trying to log into both devices with my "new" email address, and was able to do that. Trouble is:

1. When I go to Settings > iCloud, the email address showing is my old work email address, but then when I click that name/email address, and then click "Contact Information", the new email address shows.

2. When I go to https://appleid.apple.com/, and try to log in with my work email address, it says that the Apple ID doesn't exist anymore. However, when I try to add my work email address as an alternate email address, I get an error message "Invalid email - this email address is already in use with another Apple ID".

3. The photos app on the iPad is showing my wife's old photos, but I was under the impression that all my iPhone photos (backed up to my iCloud drive) would show up there. Maybe it'll take some time to perform this sync?

4. I have tried to "Sign Out" from the iCloud account on my iPhone (which is still showing my work email address). When I do this, I get a message that says "If you sign out of your account, all photos in iCloud Photo Library, etc, will be removed from this iPhone". If I proceed to "Sign Out", I get another message "Are you sure you want to continue? All iCloud notes will be removed from your iPhone". I haven't yet clicked "Delete from My iPhone".

Can anybody tell me what is going on here? I simply want to have two devices using the same iCloud account. If I click "Delete from My iPhone", what will that actually do? Can I assume that all the pictures currently on my iPhone will stay in place? Why is my iPhone still showing my work email address in some areas, and the new one in other areas?

Any other ideas?

Thanks to all for your help!

Steve