I’m optimistic that she’s gonna get a decent amount of screentime, for several reasons, but one of the main ones is that The Little Mermaid is being re-released on DVD/BluRay only a month before 306 is due to air. Ariel’s the fave Disney Princess of a crapload of people, who will be high on their new DVDs and ready to eat up anything and everything Ariel related. The character could potentially bring in a bunch of new viewers. It would be a waste for them to not capitalise on her popularity.
Perhaps they’ve only publicly confirmed 2 eps so far because they’re anticipating a whole lot of chatter in the media and social media when it comes time for the ep to air, with everyone wondering what their version will be like. Then once it does air, and everyone’s freaking out over how cool it is to finally have Ariel on the show, that will be the more sensible time to reveal that she’s gonna be around for longer. Allows for articles published after 306 to say, “She was such a huge hit with fans that we’re happy to confirm that you’ll be seeing a lot more of her in the future.” Gets everyone hyped up all over again.
They wouldn’t have gone to the trouble of shooting the promo for Comic-Con if she was only gonna be in 2 eps total.
If she was gonna be an insignificant bit part, they wouldn’t have had someone in mind to play her pretty much since the show started, and they wouldn’t have waited until that particular actress was available.