AFAIK deleting objects is not possible from nanoflows in offline. It is something that Mendix plans to add at some point https://www.mansystems.com/blog/everything-you-need-to-know-about-mendix-mobile-2.0
For now, your best option is to use a so called soft-delete. WIth soft-delete you use a boolean attribute on the object to mark it as deleted without actually deleting it from the database. Then you need to make sure that any grid or retrieves respect this boolean flag and do not retrieve objects that are flagged as deleted.
Hope this helps
That's a lot of questions :)
The recommended way to delete an object currently is to use flags (a boolean that indicates if an object is deleted, used in an event microflow on synchronisation). Mendix did announce during MxWorld that deleting objects from a nanoflow will become available out-of-the-box soon.
If you delete an object in a microflow call from a nanoflow, the object will be deleted from the Mendix database, but still exist on your mobile device's database until you perform a full synchronisation.
About the logging. Mendix also announced the debugger will become available for nanoflows soon. Until then you will need to use your own logging in your nanoflows to debug your flows.