Hello,
My name is Roman Dubyk. I'm programmer with big experience in C, C++ and C# and with processing the data including Excel automation.
I propose to implement your prototype with help of C# and Asp.Net. As Cloud platform can be used Asure or Amazon - on your choice.
My recomendation is to use newer excel file format - xlsx, then MS Open XML SDK can be used for generating aggregated file.
To show files content can be used [login to view URL] For instance, for this test spreadsheet [login to view URL], Google genarates output: [login to view URL]
You have mentioned reporting - so, depend on complexity, some additional component have to be used.
Hope to work with you, thank you,
Roman.