Hi,
I've worked on similar projects before, but the interface was a mobile app instead of an Excel sheet. I see no problem in adapting the same approach with modification to the interface layer to make it compatible with Excel.
I'm a professional IT Solution Architect and would like to approach this project in a methodical way. Following are the steps (milestones) that we'll go through to achieve your goals:
1- Understanding the requirements.
2- Design the solution.
3- Develop.
4- Test.
5- Deploy.
The first milestone is the key of a successful project. As such, I have the following questions regarding your requirements:
1- You specifically mentioned that data will be hosted on Google Cloud SQL. Is this database already setup? Is there an Google App Engine ready to serve it?
2- Which Excel version(s) are you going to use to interface with this database?