Near the beginning of this thread @Morgan wrote
As it is a deep low backless bodice there is insufficient tension or stability in the design to hold the bodice snugged into the body.
That is still the case, and I think that you will have to incorporate one or other of the fixes that Morgan suggested.
This is perfectly doable with the low back but it will need to start 1-2 inches above the waistline (just re-reading the comments). It will also require boning and also an inner laced insert (think corset-type lacing) which holds and supports the bodice and dress, whilst the zip just closes the fabric. I did a course on boned bodices and this back was discussed and someone actually did one. She had max 2 inches of fabric at the bodice back and it worked beautifully and perfectly.
I've looked at the back and I cannot see any darts. These will help pull the dress snugly to the body (especially as you are going below the waistline) and can also be the basis of your boning. For the innards, you will need canvas (I use tie canvas) on a backing of cotton lawn, which are then mounted on your fashion fabric (not sure if you have done this or not).
You have taken out too much at the armscye as well.
Don't work with lining at this point, it is detrimental to the fitting process on a toile. Just work on getting on getting the basic layer working. Get some medium weight calico, sew the toile with the seam allowances on the outside and work with unfinished edges. You can make notes, adjustments etc on the toile as you go along, adding in pieces as needed, which you can't do with what you have currently (pretty though it is!) Think of a toile as the sketches before the painted masterpiece!