I think they were still filming 6 yesterday. On Monday night the actor playing Prince Eric tweeted that his first day on OUAT was complete, and he didn’t film on Tuesday. Would he really film for only 1 day for the Ariel episode, when it takes 8-10 days to film one? If so than he’s barely in epi 6. I think that scene with Ariel coming to SB happens at the end of episode 6, unless they’re filming both 6 and 7 at the same time.
Grumpy and the rest of the dwarves, Archie, and the Blue Fairy were also filming at the beach with Belle and Ariel, so they’re definitely in Storybrooke. I think Ariel had an important message for them.
As for Belle wearing heels in the sand – I’m thinking she didn’t plan to go to the beach, but something happened and the group had to immediately run over. Though she could have taken her heels off and just walked barefoot in the sand, lol.