#1 - We added pouches where extra breast padding could go and had a belly pouch that would accommodate various size "babies". It helped to have another layer of stretch that came down from under the breasts, went over the padding and pouch and wrapped around the entire body to secure "the baby" and smooth out the look. Just make sure your leotard fabric isn't too flimsy. I would suggest buying a good tank style miliskin leotard from Capezio and dying it rather then using a flesh colored liner.
#2 - When we did Baby, we used an all in one shaper (girdle like) and jumbo spandex to make a tight pouch on the outside to slip the various babies into. We added additional 'bean bag' breasts during the quick change as well. We tried a leotard, but it was too flimsy for what we needed.
#3 - I've done something similar, but I sandwiched two leotards together to form the pockets - we made slits in the top layer like you'd have in a pillow sham. If you use leotards it will hold everything down and in place, whereas a camisole might ride up.
#4 - If you want to add to the bust line, you might want a girdle/bodyshaper that has some bust structure. We also have several different sized pads that slip into the spandex pocket during the show. The pads are working, but we noticed during the first tech that the actress was able to move the "baby" around in a very unrealistic way. We added a velcro patch to hold the pad to the leotard, so hopefully that will help. The other slight problem is that the pads seem to stay fairly high, because of the way the leotard sits on the body. Try to
make the pocket sit as low on the understructure as you can. Additionally I bought her several bras in increasingly larger sized cups which I stuffed and she could add over the leo in some of the less crazy costume changes.
#5 - My co-worker makes a spandex base or uses a pre-made leotard. The padding itself is a latex foam pillow that she carves into the desired shape and then covers with another layer of spandex to smooth out the edges of added pieces of latex. The pillows aren't exactly cheap but the result is a fat pad that moves and jiggles like real fat and is much less labor intensive than using layer upon layer of quilt batting. It also will hold its shape better than layers of batting which tend to move around and get lumpy through washing. The latex foam is washable and needs to air dry which can take some time if the padding is very big. Don't dry clean it. It will dissolve.
#6 - The idea of changing the pad is a good one. I would suggest getting the shape you want and covering it with spandex that will slip easily in and out of the pocket you make in the leotard. You can also do the same with the breast pads. Make a pocket in the leotard for a removable pad.