Currently I am using mpk files using ArcGIS .NET localmapservice (localfeatureservice) to view & edit using leaflet js library and I am doing this in offline mode.
I need to synchronize the changes I make to FeatureCollections in that MPK file with ArcGIS instance (available online) when I go online. Could you please show me the way on how to do that?
Or is that impossible & I just need to store all the REST calls & replay them manually over online instance once I get online?
Please can you clarify your architecture: it sounds like your solution might be combining multiple APIs/SDKs?
For feature collections you actually don't need to use the Local Server component (that's only available for use with the WPF API). You can use any one of the .NET Runtime APIs (WPF, UWP, or Xamarin Android, iOS or Forms) to access the feature collection online then persist it locally and while offline you can open, makes edits/updates and save the whole file locally again. Once you're back online you can save the feature collection back to you portal / ArcGIS Online.
If you are looking for guide docs/samples on saving the changes you made on your map to back to portal, you can look at these links: