Although it seems "out of character", I had attributed this feeling outburst to Willoughby's charm, and the fact that Elinor, steadfast as she may be in her principles, is still a human being. We cannot always attribute our logic to what we like and whom we like; sometimes, it is just what it is.

And, as you both mention, she may have been warmer in her praise than she actually felt, in response to the zeal of her mother.

Also, that she felt for him sisterly regard, is supported by a phrase from Chapter 11:

"Willoughby was out of the question. Her admiration and regard, even her sisterly regard, was all his own; but he was a lover; his attentions were wholly Marianne's ..."