Scrivener and Ulysses are great for some, but Dabble Writer is the most user-friendly and truly multi-platform writing tool out there. The app is actively being developed and adding new features all the time. They have a similar architecture to Scrivener and Ulysses, so I think the integration would work similarly.
Well yes, as far as I can see this is an online service. I’m pretty sure that the users’ project data is and will remain inaccessible to third-party software.