& how far in advance can I make these leaves & assemble them as a stand up tree for cupcakes
You might have already made them by now, but a quick internet search pulled up tutorials for both gumpaste (https://www.youtube.com...) and fondant (http://cakejournal.com/tutorials/editable-palm-trees-for-a-cake/) palm tree leaves. Good luck!
I'd go gumpaste because it fully dries. The leaves can be made way ahead of time as long as they stay dry (store in a container with a dessicant). I'd use modeling chocolate to make the trunks. I'd assemble them same day if possible, but I'm sure they'd hold up overnight in a cool room.