downloaded from third party site
That is just the standard Apple warning for any non-Apple download. If you did
not personally initiate the download, this is your chance to
not install it. Since you actually wanted another copy, you can simply click the OK, continue or whatever the button is called and finish the download/install.
As
Edouard mentioned, there is no need to delete the app nor even the plist file. Simply move the plist (that you think might be corrupt) to any convenient place on your drive. In fact, you can even rename it by adding almost anything to its name. Either moving it or renaming it will basically make it invisible to an app. Thus, when you try to restart an app, the first thing it will do is to create a brand new, default plist file. This is a standard process for just about any Mac app.
And, for most apps, the last thing they do is to re-save their plist files, if they have any changes in them. Force Quitting an app usually doesn't allow that process to happen, of course.
