Back to the example, if a CMS was used, what would be a real-life consequence of that? ie so that would mean a user could do abc on rewardsgateway.com but not xyz? Or information can only be served this way, but not that way to a user. Or is it more about our ability to do things with the platform, designing and maintaining it? I'm having trouble translating the characteristics of the various technologies into their impact and implications for the purpose of the platform.
It doesn't really work like that. What a CMS will give you is a lot of the backend code, such as a database, business logic and a lot of the front end for standard activities, such as a user creating a profile, updating it and so on.
Think about the standard CRUD operations for a user. In order to create a user profile you need database tables to hold the values, you need a web page to collect the data and you need the business logic to take the user's input, validate it and pass it to the database.
A CMS gives you all this for free. Without the need to code all that stuff yourself.
You will then find some that have very specific features, such as the ability to create surveys, which seems along the lines of what you're trying to achieve.
The other benefit of a CMS is that it allows you to produce content, in a consistent format, without having to hand code pages.
For a very simple example, have a look at surveymonkey.com. They allow you to create a survey, without the need to write any code. It includes the ability to add logic, so you can skip a question based upon an answer and so on.
The flip side is once you have configured your survey, surveymonkey.com will create a web page you can display to your users and it has all the standard functionality you'd expect, tickboxes, free text and so on. Again, you didn't have to code any of this.
Now, I wouldn't call surveymonkey.com a CMS. As I said, you seem to be getting bogged down in semantics, rather than focussing on the features and functionality you're trying to achieve.
My advice would be to focus on the features you want. Then do some research to find off the shelf tools which can do them. For example, this link shows
10 free survey tools. From there, you need to drill into the each to find if they offer the ability to have user profiles and so on until you have a shortlist of tools which meet your requirements.
Don't get me wrong, I'm not saying it is easy and you may well have to compromise (which is why most large web-based applications are custom built), but for an MVP this is the quickest approach.