I'm getting the sense that DITA will not worked with mixed page layouts.
Is that a fair assessment?
In many of the manuals my company produces the chapters contain a mishmash of page layout styles. Most pages have a two column layout but sometimes a spec table or image requires a one column layout.
My understanding is that it's an either/or. Whatever the page layout is in the template file you import into DITA template -- that's what you get, all of the concept, task and reference files automatically get converted into that format.
Yes, you apply one template to the whole lot, but that template can include multiple table styles. (You can have a different template for appendices - this could be another option to look at.)
My situation isn't quite the same, but I think you could achieve your desired result by applying a table/object style that spans two columns. I think you could get the style to apply based on a parameter set in the table element / image element. You may have to dig into the EDD to get the style to automatically apply.
I have different table styles applying based on how indented I want a table to be and control this based on whether I have the table in a step vs substep. Sometimes it screws up and then I manually reapply the correct style just before publishing.