1. If a script has been published (for example in the Editors Depot) then you do not need to ask the author if you can use it in a mission - he has already implicitly given you permission by publishing it. However, you must acknowledge the author in your readme and many people also do so in the credits during their Intro or Outro. The important thing is that you must not pass off other people's work as your own.
If you use a script it is also polite to tell the author by email or IM. For an author it's nice to know that the script is being used and to see it in action in somebody else's mission.
If you have made only minor changes to the script then you don't need to ask permission. For example, if it was a paradropping script and you wanted the loons to jump out more slowly for some reason, you could change the delay.
However, if you have made significant changes to the script, so that there is now some doubt over who is the real author - the original guy or you - you must ask.
If you're not sure, then ask. Always consider how you would feel if you were the original author. It is very rare of for an author to refuse permission. Some authors include general permission to change a script within the script itself. In such cases it is usual to describe it as "my tank rotating script, based on so-and-so's house spinning script."