You could also just fold the straps under, secure them with some basting stitches. Or - use a long ribbon for them. I know, it's not entirely period, but Lauren (American Duchess) made that too, as she had a gown with a wide neckline and didn't want the straps to show.
The open back - my stays are quite similar. If I like to have the closed more conical front of the 60ies, I wear the stays with the back open - if I'd need the wide front of the 80ies, I close the back and wear the front very open.

And I second Sammy, taking the bodice up when sewing on the skirts should work, you don't need to add any sewing allowance :-)

All in all: Looks good!