then it would have to be a Willoughby who has already changed for the better, and is *marrying* a girl instead of seducing and running away, he's marrying her for love - it would have to be for love and not money because Marianne doesn't have any.

Perhaps Marianne would further change him as Colonel Brandon thought.

That's why I see the situation where he would marry Marianne as impossible. He will always be;

" a liar, a seducer, a libertine, and, in Brandon's words "expensive, dissipated, and worse than both."

So letting things take its own course was the most sensible thing to do. Narrating his story after Willoughby had shown his true colours to Marianne was the right moment IMO.

After all Colonel Brandon does mention what his real reason is in narrating this;

My object -- my wish -- my sole wish in desiring it -- I hope, I believe it is -- is to be a means of giving comfort; -- no, I must not say comfort -- not present comfort -- but conviction, lasting conviction to your sister's mind.

Comfort or conviction would have played no role earlier. Anything else would put the colonel in a nagative light.