Knharde! Wow, I haven't seen you around for a while! How are you?
I don't have any advice about the cassia/amla - sorry. But if you are in dire need of a trim, as you said, I would do that first, then do the treatment and strengthen your new ends. You're hair will just feel better too. Also, although cassia and henna are good "conditioners", it's more of a strengthening conditioning. It's not the slippery, moisturizing kind of conditioning, so a good moisturizing conditioner afterward wouldn't hurt.
Also, can you tell us more about the Marshmallow root infusion that you make? I've been wanting to use some in a recipe, but I don't really know how to work with it.