Re TIE Defenders:
As mentioned, it sounds like this is debuting as a prototype model.
It's likely not in production yet.
As for why Black Squadron is flying TIE fighters instead of TIE interceptors or some other fighter type, its real simple. They probably know the generic TIE the best. It's what they've trained in and are most experienced with.
You don't want to be flying into battle in a brand new TIE interceptor when you've been using a TIE fighter for years.
