You can use it on tablets and phones via the web, and it supports off-line editing via a Chrome App. It supports PDF, FDX and Fountain formats, so that’s a way to import to it, although it might be nice if there was more direct integration. I have no idea if they offer an API, but once you’re in Amazon’s tool, there is a direct method of submitting scripts to Amazon Studios for consideration. It would be nice to have a way to upload scripts directly, and be able to import changes.
It seems to me all of the ingredients for integration are there (maybe I’m missing something?). I gave the web tool a try and managed to import an FDX from Scrivener with a single import command. Everything looked to be in order, so I made a few changes, added a scene and then saved an FDX to my computer and drop it back into Scrivener with Import & Split to break it back up into scenes. It looks to be good for round-trip to me, though I only tested a few things on a standard screenplay.