Let me try to have my first answer here
As for the first question: if all those subjects are related to 'a schema'? I believe you should make a new relationship for that more generic subject. From what I understand, that is the idea of a graph database.
As for the second one: I believe it is better to use a different database for that. Maybe something like Elasticsearch.
I have to say: I'm really new to Cayley and maybe there are ways to do this. Hope other maintainers will jump in if I'm not correct...