I've read or skimmed every post on this thread, so hopefully this response/question is not out of line.
I have a US/generic C6906 and I've had some luck installing custom roms (I'm presently running CM11 with Philz recovery). However, I'd like to dabble with the Sony experience a little bit, though I'm finding the FTF function less familiar than my experience allows.
I can't seem to find a non-modified FTF available for download (all the Mega links are broken for generic C6906 in the FTF mega collection), in order to start from a base rom as the OP requires. If I drill backward through the OP's OPs, bringing me to the 14.1.G.1.534 version FTF, but this one is labeled for a C6903. From my experience with Samsung phones, it's critical to install only ROM's indicated for the specific hardware I have. Is that the same with Sony? Furthermore, that file is still a customized "Nut" FTF, and not a stock Sony FTF.
I've read the post #32
which is very similar to mine, just from a different starting point.
Thanks in advance!