The 730WD has a triplet for double-sided: off, auto and manual. It suggests you adjust density for large coloured areas. It has a "mirror" print "tickit" and even one for 180 rotate but here's the telling thing, it greys that out on auto double-sided. And does not hint at flipping the print on the reverse pages. I am fully conversant with gutters and mirror margins, and perfect binding.

The conclusion is: my "screen dump & rotate" wheeze is as cute as it gets on this one. But I will quiz Epson on it. I await iteration 2 on that, but am not hopeful unless there is a driver update which they can offer.
Shame because I would like to print a book.