RK's phrasing of the words, though unusual, was very bitter and almost reluctant. It gives credibility to all those who call it a dark passion and Madhu would have recognised the undertones if she wasn't blindsided by the unexpected confession in the first place.
Her not referring to him as Rishab should have been understood by RK whose behaviour earlier didn't invite such familiarity on her part.
besides, she did call him Rishab when they first met but that doesn't count, I guess. also, she used that name in the hospital when he was unconscious but how was he to know that unless his mom told him..in which case he wouldn't believe her..or bituji who might tell him when it's too late!
Yes, when all else fails, there's always Dips to make things worse!