I plan on using mmf in a lace mold and than air brush. My question is how far in advance can I do that and keep it soft so I can use on a round cake.
My goal is to keep last minute assembly to a minimun.

I don't know how long it will stay soft, but to use in the lace mold you do want it slightly stiffened (air dried) for it to hold. Fondant that is too soft will not hold the mold shape. You can keep it under the flap of the practice board with a towel on top to help keep it soft. Hope that helps some!

you might be able to dry the lace on the side of the cake pan to get that shape, but i would probably go up a size to allow for the icing. ex. if you made an 8 in cake i'd dry w/ a 9 in pan.
I guess that depends on the size of your lace as well.

if the whole cake is covered in fondant, i have just laid all my accent pieces on top & let them dry in the right shape, & they lift right off later & keep that shape.

Good Luck!