I'll only respond to the question about how to "test" .launch files and ignore the other questions you've included (note: it's not a good idea to include multiple questions in a single post for various reasons).

The main problem is that we write a lot of launchfiles and we inevitably have some errors, like a missing paramter and I was not able to catch them automatically.

For this specifically, you could use roslaunch_add_file_check(..).

See wiki/roslaunch: Catkin.