Nicolas, Integration testing is not my total expertise, but hopefully some others will put there thoughts in here...but here are my thoughts. I do not think its a bad idea for setting up a separate server for unit testing, but are you using version 9? If so, I would not think about staging but instead would implement the new continuous integration module which will give a much more accurate sync of changes and can be done in only a 1 way direction if you desired. Plus you have granular control (if you want) around what you want to sync, such as not user accounts.
As far as the 2nd part, my suggestion would be either as you create the users, you either store their ideas in an in-memory list of some sort so you could just delete out those records, or maybe create a 2nd table for that which you could query for all those values. Where you could just use the creation and updated dates, you run the risk of deleting things that were unexpected because of that.
Just my thoughts.