A: In your position, my main worry would be losing the impetus that I'd built up during my training, so you're right to want to stay in schools. However, I'd keep well away from anything that had the smack of admin around it. You need to be working directly with pupils, and being a cover supervisor would limit your opportunities. Being an art technician would bring you nearer to the classroom and would give you opportunities both to observe and to introduce new techniques.
If I were a potential employer, I would be looking in your CV for evidence of your passion to work with children and to understand the way they learn.
You could do that as a supply teacher, but your subject experience might be random.
If, however, you found work as a learning support assistant you would be gaining a whole new understanding of learning from a fresh perspective.
That would give you opportunities to use your talents as an artist. You could look after displays, help with drama productions, and your versatility and willingness would quickly be noticed.
Subject knowledge, in the context in which you frame your question, would look after itself, provided you kept up your interest. I'd go for classroom experience.