The answer is you can do it. Here are numerous discussions showing what others have done https://community.appinventor.mit.edu/search?q=change%20order. There are many different ways the order of Screens can be changed. The only thing missing is code showing what you have done/tried.
The simplest way to have a Splash screen in an app is to make Screen1 the splash screen from the very beginning of coding. Developers can use virtual screens Building Apps with Many Screens too.