Try setting a breakpoint after the ExecSQL task so that you can inpect the runtime contents of your vSQLString variable ... perhaps that will throw some light on what is...

What is the data type of @[User::vDateThreshold]?

When you say: "I can see that it isn’t evaluating my variable correctly", what are you seeing, exactly\?

Yes, to examine what SSIS packages do, you need to open them in VS/SSDT.

I thought that IMEX=1 was valid for reading files, not writing to them. 

What problems do you see if you omit it?

CSV files are text files. They do not have sheets.

Three separate data flows, in series, to the same file, should do it.

There are no technical reasons why this would cause problems.

But if your spreadsheet users change the schema of the spreadsheet, as users tend to do, those changes will...

Are the list items held in a database table?

What does the output from 

select