I tested uploading an epub directly to the KDP, as others had advised me. It converted fine except for one thing: the Go To / TOC option no longer worked in my Fire. (I don't recall whether I tested it in my K3.) I quick replaced that upload with the underlying html file, and behold! Go To / TOC worked fine again.
I don't use OPF/NCX files except as they may be generated by Sigil.
Would this be a related phenom to what's being discussed here?