The sheet metal flatten command will not work because the bend is not in a single axis.
What should work is using the surfacing type flatten command. The surfacing portion works from polygons rather than constant radius sheet metal bending. I can't help with using that as I have never used it, but I'm aware of that it can flatten complex geometry like this.
You might try changing to a part (file/switch to/part) to flatten. I use ST8, and have many parts that are not "traditional" sheetmetal parts, and I switch between part and sheetmetal to flatten.