Soute..,
I have done alot of editing the main campaigns during testing (as well as some of the other earlier SC games), and can tell you that usually when you choose to manually edit the scripts, it usually doesn't actually freeze, it simply pauses (sometimes for several minutes) before you are finally able to move your mouse to edit the scripts. Just be patient. It may be better to just choose "save without editing scripts" in my experience.
Second, any time you choose to edit some features in the campaign, it lead to unforeseen problems with the scripts and can cause some scripts, DEs, and even unit problems that are difficult to foresee until after the fact. If you don' t have a lot of computer savvy (like me), just use trial and error until you get the results you are looking for. It can take a number of false starts before you get it right.
As they say, practice makes perfect.