Yes, you can. Nanowrimo doesn't restrict you to just one novel. If you're feeling creative and have the time and energy, you can work on multiple novels during the Nanowrimo period. It's all about challenging yourself and getting those stories out.
Definitely. NaNoWriMo doesn't strictly prohibit working on an existing novel. It's all about the effort and the drive to make your writing project move forward, whether it's a new or ongoing one.
Not necessarily. NaNoWriMo is about challenging yourself to write a substantial amount of words in a month. While a novel is a common choice, you could potentially create other forms of long-form writing.
Yes, you can. Many novels have multiple antagonists. For example, in 'The Lord of the Rings', Sauron is the main antagonist, but there are also other evil forces like the Nazgûl that oppose the protagonists. Having multiple antagonists can add more layers of conflict and complexity to the story.