I agree with your that for first-time readers of S&S Willoughby's true character is not obvious during the Barton chapters. Sure, there are many hints here, but most of them are ambiguous.

For example, Cowper etc may have been 'disregarded before', but is W. 'lying' here? Would we really expect a man who was not a great fan of Cowper to respond to Marianne's enthusiasm with the unvarnished truth?
("Cowper? No way! Gimme "The Monk" and the "Racing Times!"" Hmm... I don't think so.)
As Barbara points out, there is a real commonality of interest between M and W over music and dancing, and a shared "ardour of mind".

Once we know what a louse Willoughby is, on re-reading all the ambiguous hints appear as pointers to his real character. It is so long since I first read S&S that I can't remember my first impressions, but I think JA intended us to share Elinor's point of view, beginning with happiness for Marianne and liking for Willoughby mixed with a few minor criticisms. Gradually her niggling doubts become real unease, but it is not until the brush-off and Brandon's revelations in London that the shocking truth becomes clear.