I am so torn on this I almost didn't answer (my grandmother had Alzheimer's), but I feel I should at least say a little something.
First, I admire your courage in finding practical helpful solutions that will work for both of you. There is a lot more stuff available now then used to be in the past, and you should be able to find things which do help a lot.
Second, I think your basic idea is really good, and I think you can probably even simplify it further. More about that in a minute.
Third, I would consider upfront how reliable you need this to be. I myself and quadriparetic, use a wheelchair with Limited hand function. When SmartThings works well it is absolutely my favorite home automation system. But since last November I have yet to go nine days without an impactful failure. It's just not reliable yet. The company is very aware of this, and even their CEO has committed publicly to improving this aspect of their system. I do believe that they are sincere in that, but they just aren't there yet.
This may not be an issue for your family, but I know with my grandmother routine was very important to her. If she had had something where the light was supposed to come on and then it didn't come on we would've been dealing with the repercussions of that for days. If reliability is an issue for you, I think you would be better off solving this problem without putting SmartThings into the picture at the present time. ( if you search the forums for "reliability" you will find much discussion on the topic. )
So since you asked about "major pitfalls" I think the reliability of SmartThings has to be put into that category at the present time. The following was posted to the forums by the company CEO in April, and there have been at least two significant problems since then (one this week).
The second possible pitfall is complexity. There is a very sophisticated community-created rules engine in beta now called core. I'm sure it could be designed to do all kinds of things that fit your use case. But I'm going to recommend that you not go that way at the present time for the same reason. If I were in your situation I would put reliability at the top of my list. I would look for the simplest possible solution.
You may feel very differently, and it may be that your relative's cognitive issues are such that reliability isn't that big a deal. He might not even remember that it hadn't worked the day before. Alzheimer's is a very individual condition. But I just wanted to mention this so that you can take into account. I think core is a great piece of software, but it can't be any more reliable than the SmartThings platform overall. And the fact that is still in beta adds a little more unreliability to the picture. So I really like it, and if you were an IT professional who enjoys tinkering and was willing to put up with occasional failures I would encourage you to use it. But it doesn't sound like that's your situation.
a possible alternative
My suggestion instead would be that you get a Phillips hue bridge with a few of their $15 "Hue white" bulbs. (By the way, this is zigbee, not Wi-Fi, but while there's a technical difference you won't really notice it in this set up.) this is very reliable equipment, very consumer friendly. If you think a color changing Bulb would be of help so that you could change it, say, from Orange to blue for different purposes, then you can go with one of the more expensive RGBW bulbs. But I would think the hue white would be enough.
Next, instead of trying to use a pressure mat I would just get a good quality camera with a built-in motion sensor and a built-in two way intercom that also "has an IFTTT channel." There are several of these.
IFTTT is a free service that let you set up "recipes" so that an event on one device can trigger an event on another device. In this case, you would have motion detected on the camera trigger the lighting scene to come on on the bulbs.
If you haven't used IFTTT before, there's a good tutorial online. But it's a very simple service and again, very reliable.
Now you just have to control the field of the motion sensor part of the camera so that it only detects in the range that you wanted to detect. You might put a tube on it to narrow the field, you might use tape, there are several options. But you just have to position it in a place and a way that you catch motion exactly when you want to. And where the voice coming from the camera will not be frightening to your relative. (I mention that because sometimes people put a motion sensor for this purpose underneath the bed, but I think that might be too confusing to someone with Alzheimer's. I would look instead at putting a camera on the nightstand but maybe pointing mostly towards the wall. You'll just have to experiment with how you want this all to work. )
If this had been available when my grandmother was still with us, I think I probably would've set it up with two cameras one right near the entrance to the bathroom and one right near the bed. But there are a lot of options.
Because the Phillips hue has scenes you can set it up so that when One of the cameras is triggered the light comes on and then goes off again very gradually over five or 10 minutes. You might use that for example if the bathroom camera was triggered to control the bedroom light if that makes sense.
Anyway, you just have to experiment to get the timing and the positioning right. But you would only be relying on two types of very highly rated devices, a particular camera and a Phillips hue bridge. They would be linked together with IFTTT. And I think it would do everything you want.
Several of the available cameras also respond to noise, not just motion, which means if your relative is lying in bed and wants to contact you they just have to speak up. That can be useful also. When you look at the reviews, look to see if people are using the camera as a baby monitor and that might have features that also fit your use case.
Oh, I should also mention that with IFTTT although it isn't obvious when you first look at it, you can have one event trigger multiple recipes. So you could have the camera sensing motion turn on the Phillips hue lights with one recipe and also notify you with a second recipe. So even though there's no "and" in an IFTTT recipe you can get the same result by using two recipes triggered from the same "if."
Anyway, that's how I would approach it. It would be a very simple system, but I think it would be very reliable for you, and it would still give you some options just because of the versatility of IFTTT.
IFTTT is a cloud service so you do have to have Internet on, but in my experience it has been very reliable and I've been using it for almost 2 years now.
There are certainly much more complicated things that you could do and, as I said, maybe reliability isn't the top priority in your situation. If not you could definitely go with SmartThings and the kind of equipment you mentioned in your first post.
So just some thoughts which may or may not be relevant. Good luck with your project.