My photos stopped going to the iCloud in February. When I logged into iCloud.com, there is nothing after 2-28-15. I went to the Apple Store, and the genius guys said "fascinating." They checked all kinds of things, and couldn't solve it. My phone is 2+ years old, and I am due for a new one. Will a new phone solve this problem?
It's unlikely that adding another device to the mix will help.
Did the genius explain that AppleCare over the phone supports iCloud for issues like yours and mine?
If you have an iPad or can borrow one, you can test adding a new device to iCloud without needing to buy an iPhone.