Droopy shoulder problem is fixed by using a shoulder pad in between the fabric and lining.
harbinger Resizing a dress from big to smaller is actually very technical, as it's not as simple as taking in at the sides. For example on a larger size shift dress, the shoulders are usually cut wider, the armholes are cut lower - you can't put that fabric back. Bust darts are lower on bigger sizes, and if the dart fabric has been cut away on the inside then you can't reposition them (if the fabric was not cut but pressed flat you might stand a chance.) Taking in dresses only usually works within a couple of sizes and even then a lot depends on the style. In the example used in the programme, the dress was very loose fitting to start with, plus it was slightly stretchy, plus it was not lined.
Stoop to back disguise would depend on the severity of the stoop. With slight stoop, extra padding can be sewn in but anything significant wouldn't be completely disguised without making the garment too bulky.
LRD for great tutorials on things like cutting patterns then Tilly featured on the programme has her own blog, Tilly and the Buttons, and she explains basics extremely clearly in her tutorials. (I am not Tilly BTW!)
Tunip you can't really fit a top to wonky boobs
it would emphasise not hide! You would need to make the boobs right from the inside ie a filler in one bra cup. Then make the top as normal (symmetrical). There's no way a top with different darts to shape to the different sized boobs would look right!!!!!