Regarding your first point: Decentralized social network "reader/consumer platforms" would be commercial products. They would provide you good UX and easy ways to filter information according to your needs. That is why such model could succeed. RSS was a failed attempt because there was no singular decentralized database where anyone could fetch all the RSS feeds of the world.

It is totally possible that some DSN readers would run ads and some other would have monthly subscription fee. It would be healthy competition. What is important is that they would all be built on top of one singular decentralized social graph.