Fondant will stay soft if kept sealed in a plastic bag for several days and maybe even longer. If it were me, I would start now and not care if they got hard. Most people will probably not eat them anyway.
I'm with Sandra. I would start making the leaves now and not care if they get hard. And anyways, if it was me I would want the leaves to get hard so that way I can lay them over crumpled paper or something similar to give them some movement and they would need to get hard to keep that shape.
for the fall -- I would want colored leaves, changing autumnal colors meself -- at least have them tipped in another color or something maybe 
Ok, -K8memphis and kakeladi sparked another idea, how about edging a few of the leaves in gold?
actually my favorite is chocolate leaves made/brushed on real leaves and edged in choco
I mean chocolate leaves edged in gold like you said, sandra — duh k8
I knew exactly what you meant, -K8memphis! I could see them in my mind's eye!
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