1. Two different apps:
App1: createt completly with Crosslight and offline data synchronization
App2: created with Xamarin or Xamarin Forms.
App2 needs the same offline data like app1.
--> possible?

This synchronization service has been designed in a way so that the data synchronization process can work seamlessly between client and server. On the client-side, Crosslight has provided complete features so that the implementation of sync service can be made easy and simple.
On the server-side, we also have provided its framework. For more detail information, please refer to Enable Data Synchronization article in Crosslight documentation.

If you want to implement sync service without using Crosslight, then it will be very tedious and difficult because you need to set up a framework which is equivalent to Crosslight.

2. One App: UI/GUI created with Xamarin Forms, logic (bindings, views, data sync, etc.) with Crosslight
--> is this possible?

I'm afraid that this is not possible for such scenario because Xamarin Form is built with Unified View pattern which mixes UI definition. It is recommended to use Crosslight Form Builder that is built with MVVM design pattern where the data input is provided in the Model and ViewModel, which are then bound to the View in loosely-coupled manner.

Hope this helps.