Read your post and I wonder about durability with lichen - it will, absolutely, dry out over time and become very 'crumbly'. I use it all the time to enhance scenery but I put it in resealable plastic bags when done and sprinkle with a few drops of water and that keeps it pliable, lasts for years.
Agree there needs to be a better way to adhere foam to Woodland Scenics armatures, the plastic ones any way. Don't recall that being such a problem back in the day when the armatures were metal.
I think you might have better luck over time with the Poly Fiber from WS than using lichen.
Deciduous trees are harder to get right than evergreens.