How much more of Ellaria and Sand Snakes will we see?

It is pretty confusing, agreed. Ellaria must be the one who will be the "diplomat" of the bunch because it is hard to see any of the others pulling that off. So maybe she travels to build alliances, or maybe others travel to make alliances with her (less plausible IMO) and she stays in Dorne, somehow with undisputed leadership even though she is a bastard and not even a Martell bastard so has less than zero claim on Dorne's leadership....

Obara and Nym are in KL, yes. It looks like Tyene being planted with the Faith as in the books may not happen because she is NOT in KL. Obara may not stay as her role is probably eventually elsewhere (direction Meereen?). Nym would be recognized by Jaime, so if she stays in KL she would need to do so covertly.

I still have my fingers crossed that we will get Sarella in season 7 when the Oldtown action gets going. But if not, Tyene's short hair may be partly to set her up as a "boy" novice at the Citadel and take over Sarella's role. This season, however, I guess she stays with her Mom?

Are they even in season 7?

I suspect at least some of them will be in season 7. 

If you think about the books, there is 1 Sand Snake in Oldtown interacting with Sam and in place to have some sort of role in the Oldtown storyline when Euron invades. It is very very unlikely that Euron will attack Oldtown until the end of season 6, and Oldtown characters were not cast. So Oldtown action will be in season 7, and at least 1 Sand Snake is likely involved.

In other storylines, Dany's arrival will not be until the very end of this season if it happens this season at all, and at least one Dornish character will need to be around when that happens because Dorne will no doubt figure in how her arrival plays out.

I suspect there are other ways these characters interact with main storylines as well, but that is all we have to go on from the existing books.