It really seems like I need an independent source of the complete data set that I can make edits to and point Tableau to that source.
Yes, your analysis seems correct to me. And I'd suggest you use SQL as that source. But that's just me. Others might disagree. I know Jonathan has a lot of his stuff set up on Access.
Thanks for detailed analysis and explanation. Based on your data volume and regular process, i suggest sql too. I do have something similar for a marketing analytics and dashboard solution. I use ms sql and have a SSIS process to load data and validate and fix any issues.
My 2 cents and good luck.
I have a very similar situation where i have to do monthly updates from a csv. I have figured out how to set tableau up in a manner that allow me to clean the data much easier and faster than doing it in excel. But from there i basically do what you describe in #7 and this works fine, but I'm in the same boat as you searching for a more efficient work flow.