I worked along a bit on the beast shaping. It appears that in the cloth room you can use a 'mandril'  object under the skin as collision object, so I have been experimenting with spheres and with a transformed copy of Antoina's own breast which are then moved during the simulation. It is a definite improvement over my earlier attempts where I tried to do with action of gravity on the skin only. With the support from the inside, the skin does not collapse anymore, and that was my biggest problem. 

There is definitely a long way to go, but here is a first result. The left breast is sagged just a weebit to make it more natural in unsuspended condition. More would hurt her feelings. Yes I applied some fairing with the morph brush. 

What jumps to mind is the possibility to use a shrunk version of the cups of the clothing as mandril, and so push the breast into place in conforming clothing, or use same mandil to shape breast and conforming clothing and so ensure fit.

added:

Using a ribcage, like Thalek proposed, will also bring improvement. In fact you go to skin that is plied over a rigid internal skeleton.