I have scratched my head at the naming and grouping of these products previously as well. From what I could figure, every now and then the groups change. As you have noticed rightfully, EI was the latest one to change. It indeed split into 3 entities. It is however still usable with code from EI 6.6. The Micro Integrator can be downloaded separately and used with all the code and CAR files made with 6.6. I have run a couple of them and found no problems thus far. It is basically the latest version but without the Carbon framework and frontend. One MI instance also runs just 1 CAR file but does so really fast.
As for your DSS needs I have no experience with that side of WSO2. I googled however and found that DSS can be downloaded right here as a standalone product. I hope that solves your last question.
The APIM is not superior to EI either, but functions in a different way. Since the older products were all based on Carbon as you mentioned before means there was a certain bit of overlap. IS for example can also be downloaded separately.
If I was to guess I would say they WSO2 will be trying to make all their services into Microservice versions in the upcoming years. This is just speculation btw.
If you want to dive more into the reasoning behind WSO EI 7.0 and why it was decided to work like that I would suggest this talk by the WSO2 director.