No. As I understanded, when Aule questioned Yavanna, what sort of trees she want defend, she answered that one can sort defend theirselves (ents), one can't - usual trees. Spirits was only spirits, they had no body, they lived in trees and guarded them. And not all time of Arda - at third age they wasn't there.
Just don't use the lore to claim that Treebeard wouldn't fight when you're also having the Galadhrim knocking down woods and building siege machines. Maybe they did that sort of thing in ages past but not by the time of the WotR.
Maybe. But siege wars versus Sauron was also in third age, when Gil-Galad and Elendil, Isildur, Elrond, Kirdan and others fight in Last Alliance... So there can be siege weapons.