Thanks for your time spent on this.
I removed both trees from Ancestry so was then able to upload (directly from my master FTM tree) my slimmed down tree without any media files and without linking it to FTM. The idea being that once that's done, I can produce my second full tree with full media and FTM linkage. But, the first slimmed down tree also displays all foreign characters as a '?'.
So, the foreign character problem has nothing to do with the export+import, it seems to happen whenever I upload a non-linked tree to Ancestry. Which is essentially the same issue you're seeing. Wonderful. Now what?!