The part of your question I'm unsure of is whats happening on the web client (which sounds like it sends data?) to the GP Service which in turn gets turned into excel? Theres multiple ways to interact with GP via the web in regards to input parameters. I dont want to suggest 1 way as ultimately it depends on how you have data and how the service will accept it. For this part of your task your web development and task (python script) development will have to work together. You'll probably need to build an app that is capable of sending data in an expected way to the gp task. OR you have to build your python script to accept data and possible massage it so you can then convert it.
Hope these ideas move you forward some.