Hi! i used the billowing technique on a purple cake in my gallery. I used fondant and tylose, based on my experience and the look I prefer I wouldn't make them in advance, just because I was going for a flowly look, not a more structured pattern.
I've made one billowing cake and I used a 50/50 mix. I find you can't really do them in advance as it doesn't allow you to fill in between the layers properly once they are dry. I add one billow and then depending on what I'm using in between each billow (brooch, candy, etc) I use that to press into the billow as a guide so I know exactly how the next billow will fit. It worked for me.
Here's the link to my billowed cake so you can see what I mean. I hope this helps.
I've used the billowing technique on a couple of cakes and I would say probably not. They would dry if you make them in advance and you want to be able to manipulate them a bit. Also, I made mine out of straight fondant with no problems. I will say I let each square "breathe" a bit before each step so they wouldn't be too flimsy. It wasn't a problem to do so though because I was making several at a time.
I used straight fondant, made about a week (maybe 10 days) in advance, kept in covered container. They remained soft enough to manipulate but firm enough to hold puffed shape (if that makes sense). Worked fine for me.