I am somewhat new here but I will do the best I can to address some of your points.
It is true the SOLID project is to allow users to gain possession of their data, but the inclusion of RDF-style formats for use in navigation and storage is to allow the easy injection of context into data, and also allow for users to swap pod providers, identity providers, and other digital agent-based services easily in the long-term. This is vital as data without context is just a string, number, or other type.
It is supposed to be an alternative/competitor/replacement to existing structures where large providers hold, own, and sell all of your data. Controlling where your data goes, where it is stored, and who has access improves users’ privacy and choices regarding personal data storage and use.
As far as social media applications goes, there would be some changes. Determining where posts are stored is a challenge - how do you verify if the post has been changed or deleted? But also would provide some benefits. I can add other URIs that are blogs to follow, and could also remove them as I see fit, and as of now there is no algorithm to show you things you’d like to see - it would be relatively organic. You could also potentially have a co-owned pod for multiple agents who post data to the same URI, and users could follow that - though that brings challenges about who handles the ability to read from and controls that feed.
As far as answering questions of claims about an adult, I would recommend reading this specification. It will give you a better idea of how that question should best be answered currently.
Encryption is important and SOLID servers are working towards it - but that would be better asked to the maintainers of the Node SOLID Server, the Community SOLID Server, the PHP SOLID Server, and Inrupt’s team.
As far as copyright goes and data being plagiarised, that is an existing issue as well, and I have seen nothing so far to address it, so I hope more informed people chime in. But that is currently no different where a large company right now takes my data and sells it, even though I am the one who produced it.
For the Pod provider establishing identity, there is no need currently. The Identity Provider is the one who handles your WebID profile, and other services authenticate against this. Adding more data to the webID is possible, such as links to keypairs, tokens, and certificates for approved applications has received some attention.
If I haven’t addressed enough, or am incorrect, please read this thread right here. It is full of information regarding the future of SOLID applications, as well as some facts and opinions about the current state and where the SOLID specification should be going.