Hey @hubert,
Have you taken a look at the Sync Gate Guides over at the Mobile dev portal?
Regarding some of your questions…
1 Couchbase Mobile is a great solution to distribute content across multiple and different platforms. For Attachments, they are not part of the document’s JSON object but instead are uninterpreted data that is associated with a document.
2 There is a feature called, Channels, that would allow you to partition the data set accordingly so that your device can get part of the data set.
3 You can get the entire dataset
4 You may through the Replication class. Take it look at the Replication API and the ChangeDelegate method