Just being straightforward here - You sound like yet another person who is trying to get a 100% guarantee that nothing will go wrong if you use the STunlocker app and if you mess up you want a 100% guarantee that you can fix it yourself.
TBH: that's not gonna happen.
Either you accept the responsibility and risk that there is a possibility that you might mess something up, or you don't use the app and don't worry about it.
Shimano E8000 ST Unlocker..now bike doesn't work - EMTB Forums
Read that thread above. Somehow that person messed their e-bike up and immediately assumed it was the fault of the STunlocker app (pretty sure it wasn't).
If any of that scares you, then don't bother using STunlocker and just stick to the Shimano app and the features it gives you.