I am having 2nd thoughts about the S5. It can do wonderful things, but it became obvious early on that operating it is not intuitive.
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Anyway, if we keep these phones, I foresee a long frustrating learning curve both for me and her.
The one major fault with Samsung's flagship phones is that there's so many features. Most of which you'll never use or are redundant ones that do the same thing that Google already built into Android. Samsung throws them in there because they know that's what sells phones - lots of features. But the trade-off, which you're finding out, is that all this extra junk just makes things more confusing. And unfortunately, the settings and menus on the Galaxy S5 (and Note 4) are an absolute mess. I know Android nearly inside and out, but Samsungs throw me a curve because nothing is where it should be and it's far from intuitive.
If you decide to exchange your S5's, I would suggest looking at Motorolas. I don't know what provider you're with, but the Moto X is available with all of them, and the Droid Turbo (Verizon exclusive) is a beast of a device. Motorola sticks with a near-stock Android "experience", which is much simpler and less convoluted with unnecessary/unwanted features that confuse you. Let me clarify: Android is an open-source operating system (being Linux based), meaning anybody can use it for free. What Google produces is known as the Android Open Source Project (AOSP), also referred to as "stock" Android. This is what Google uses on their Nexus devices. Manufacturers take AOSP and modify and build upon it. Samsung probably does the most modifying/adding. LG is pretty "heavy" too, as is HTC. Motorola however (having been briefly owned by Google - now Lenovo) keeps things relatively stock, with a few pretty cool added features.
I rarely do this, since I have scruples with Apple, but perhaps you and your wife might be better off with an iPhone 6 (or 6 Plus, if you want something gargantuan). iOS is pretty simple and easy to learn. Android fanboys like me poo-poo iOS for the lack of customization and depth of capabilities, but if all you want to do is make calls, get on the internet, watch some funny cat videos on youtube, do emails, then iPhones do that easily and well. Great cameras too.
One thought I have regarding your wife, look at the Note 3 or even Note 4 (more expensive, since it just came out). It's a big phone, so that might be off-putting. Other than its size, the Notes' key feature is the S-Pen - a proprietary stylus that fits inside the phone when not in use. It works as a "finger", but with more precision and added functionality (there's a button on the side of it). That way your wife can pull out the S-Pen and tap away with ease. You can also set it up so if the S-Pen gets too far away from the phone (like it was set on the table and forgotten), the phone will alert you and remind you to replace it in the phone.