Isn't it kinda hypocritical to say Timmain needed to "borrow" the Palace's energy to willingly shapechange back to her elfin self then also think all elf magic is strictly a matter of spirit so Winnowill can do anything she wants to without a body?
First, as a High One, Timmain is far more powerful than Winnowill and she's been willingly changing her shape for untold millennia. If she consiously wanted to change her shape to talk to her 'children' she could have done it at any time after Cutter fell into the trolls' pit trap. Or when she was so desperate to enter the Palace she clawed and whined at the shield wall. She only tranformed after she passed the Palace's portico. After which Treestump proclaimed, "...And now the Palace has returned her to us--as a High One!" I don't brush that off as a simple aside 'cause the Pinis are (were) pretty careful about language and dialog and how the two progress a story.
That Timmain was (rather) unconscious of her status change is easy to see in the subsequent dialog. The elves' first communication with her was Suntop's sending. But after she transformed she spoke--verbalized--not sent. And when she did talk, she spoke of herself in the 3rd person.
"Time...the wolf could not mark it's passing...she--I...was
And she didn't send until Strongbow (the purest of senders) sent to her. He saw that she was still wrapped up in the now of wolfthought and slowly working her way back to the endlessness of elfin thought.
I still say Winnowill (and other bodiless elves) need a physical base to work true magic (meaning matter and energy manipulation beyond themselves). Winnowill meant to infuse herself into the Palace shards to wreak maximum damage. The only ones who were ever vulnerable to her on the astral plane were those who had left their spirits open to her there. The Gliders didn't immediately head for the Palace when Blue Mountain fell. They wanted and needed a physical nexus to maintain and direct their energy to a single purpose. Someone strong enough in his own right to manage that.