Can I make changes after I launch a mission?
Yes you can!
You technically can't edit a launched mission, but you CAN close it, copy it (by selecting copy from previous when building a mission), modify it to your heart's content, then launch your new-and-improved mission.
And since you won't be prompted to pay $295 until you invite scouts from a dscout screener to a mission OR invite more than 5 outside recruits to participate in it, you can effectively experience your mission before you pay. For details on how to do this, go here.