I would not make another payment until the pool is completed. I am assuming this is a large company so maybe they don't really care if you pay or not, but it is your leverage either way. I'm sure you have already done this, but I would ask for a firm schedule of completion as you are well beyond the contracted date. You might even want to do this in person.
Regarding being put on the plaster list in the beginning of June, it seems to me that nothing is being lined up ahead of time. Maybe they were waiting until the tile was done before they even scheduled it. I'm not saying that it is right, but this may be the case. I guess it's also possible the plaster company says, "Call us back when you're ready and we'll get you on the schedule." That is sort of the impression I got for ours in that we weren't really "on the schedule" until everything was done, sort of like those restaurants who refuse to seat you until the whole party is present (I HATE those places...).