About the Elite 4, we do know that Heather and El are part of it...
The third member is unknown, but from as far as we know, yes, she is.
Victoria did appeared briefly during episode 15, when we're going down the waterfall to rescue Amaria and Titania, so she'll probably return.
I don't think we need to battle the gym leaders' substitutes, since by the time we adquired the badges they were still alive (Corey never gives you the badge, and you only receive Kiki's after her death.)
Cain and Fern did met once, since Fern had the key to the wasteland. Apparently, it didn't end well by the way Fern describes Cain.