I think different people handle this differently so there are a lot of creative ways to approach it.
Just to give a little perspective from the other side…
I'm quadriparetic, in a wheelchair with limited hand function. I have a housemate who works irregular hours. I have aides who come in a couple of times a week. I have a service dog. So there a lot of times when it's just me and the dog at home.
I have a panic button that I wear all the time. I really like it. With one button I can connect with two way voice to their help center. So they can call my neighbor, or my housemate at work, or 911 depending on the situation. The device cost about 50 bucks, with a $20 a month fee. No contracts. It's waterproof, and can be worn in the shower. Works on the Verizon cellular network, which is a good coverage where I live. It also has GPS, so if I use it while I'm out in the neighborhood they can locate me.
There are several brands of similar devices. The one I use is the great call splash, which is the waterproof model. You can buy it at RiteAid or from their website. Don't buy it from Amazon, because it messes up the customer service.
It doesn't integrate with smartthings, but I don't care. It gives me everything I want in a panic button device at a very reasonable cost.
So I guess I would say that's the first thing. You don't have to have all your systems integrated together. The most important thing is that you cover all the situations that you want to cover. The panic button device give me something that I can initiate from my side no matter where I am, at home, away, in the shower. And, it doesn't just call 911, it gives me a person to talk to who can call my neighbor if all I need is somebody to come help me with something simple. (Classic example: some idiot delivery person who left a package blocking my gate so I can't get out.)
As far as motion sensor monitoring, or contact sensor monitoring, sure whatever gives your family peace of mind.
There's a topic on a medical smart home that's being developed for VA patients with traumatic brain injury, and it might give you some more ideas.
p.s. The smartthings presence sensor device is not waterproof, and cannot be worn in the shower. Even if it could, you would probably lose the signal there, so it would indicate you left the house, the opposite of what you want.
Its purpose is really for geo-fence detection, that is, you left a 100 m radius area, or you returned to that area. So perfect for attaching to car keys, or a backpack you always take with you when you leave. Many people leave one in the car's glove compartment or attached to the visor on the windshield.
The panic button devices are different, because the person usually triggers the call from their side, and because you have two way voice communication. So those are the kinds of things that you wear all the time. (You can also pay about $35 a month instead of $20 a month and get fall detection, where the device will initiate a call automatically. Useful for some people, not worth the extra money to me.)