Regarding your first point, it is quite possible that by then the rumours had outstripped the truth regarding Wickham's debts. People do love to gossip and after Lydia ran away, every trademan in town was telling stories of how Wickham had cheated him!

"All Meryton seemed striving to blacken the man who, but three months before, had been almost an angel of light. He was declared to be in debt to every tradesman in the place, and his intrigues, all honoured with the title of seduction, had been extended into every tradesman's family. Everybody declared that he was the wickedest young man in the world; and everybody began to find out, that they had always distrusted the appearance of his goodness. Elizabeth, though she did not credit above half of what was said, believed enough to make her former assurance of her sister's ruin still more certain..."

Also, Mrs. Gardiner says in her letter to Elizabeth that Mr. Gardiner would most certainly have done all the paying if they hadn't believed Elizabeth to be in on it. I believe her here - she isn't one to say such things just for gratitude: