No, there's no data going anywhere if you don't export it
Only if you need to download a species list or one of the alternative layer systems. You can't look at maps if you are offline unless you have a cached map.
Not in the near future, sorry.
No, photos need to be handled separately.
Vegapp does not distinguish between plots and plot observations but you can assign a "Permanent Plot ID" to your observation
You can ask via greens [at] vegapp.de but we cannot promise anything. The chances are higher if you have a synonymized checklist along with a valid citation for the list and a license that allows that usage.
Yes, you can use this app for professional or commercial purposes - at your own risk, it's originally intended as a research tool and for teaching.