How to decompress and run .app?
You are asking the open command to open content from stdin.  open can be made to read from stdin but it will open the contents in at text editor--which isn't what you want. 

Try using osascript directly:
If the zipped file is simply AppleScript, then you accomplish this with the following.

unzip -p file.zip | osascript - 


From the osascript manpage:


  osascript will look for the script in one of the following three places:
  
  
  Specified line by line using -e switches on the command line.
  Contained in the file specified by the first filename on the command line.  This file may be plain text or a compiled script.
  Passed in using standard input.  This works only if there are no filename arguments; to pass arguments to a STDIN-read script, you must explicitly specify '-' for the script name.