If you add a trip to the pass in the future you can always delete it. If you add a journey for the actual date it can´t be deactivated anymore :/

Example lets say you add a trip for the 16th May you can then delete the day till the 15th May 23.59 :)  General advise i add my trains usually just minutes before boarding or directly after boarding to the Pass :)

The pass is only activated in step 3, where you choose the first travel day. You can also cancel the first travel day.

The trip can always be edited: add or delete journeys.

But the best is not to use the rail planner app for planning, since it doesn't get enough updates to be reliable. Use national planners or apps and for international planning, use de DB planner or DB Navigator app.

Do you mean not to use the rail planner app when looking for trains to board when you’re actually ready to move to a different country? Or just for planning a trip. 

The latter. The planner in the app should really only be used to fill out My Trip, i.e. as a replacement of the travel diary accompanying the paper pass.