Naira learns from her past mistakes and did the right thing by stating the fact to Karthik about Mansi. At least one person from the Goenka family needed to know the truth.
Karthik was bang on about not telling the family as it definitely wasn't some great thing which Mansi has done and needed to be applauded.
But the place where Karthik faltered was hiding it from the parents of both the parties. The parents needed to be bought into the loop to avoid the blame game and confrontations which would take place during the wedding when this news would get leaked.
And as many people stated why was Karthik getting such costly rings. It was clear. Karthik was buying Rukamini. Kqrthik is well aware of Rukamini's greed for money and the power she holds in her house / family .
After getting to know the condition of Mansi, he is assuring that this marriage gets over soon and that Rukamini keeps pushing the Singhania family by seeing so many riches. Moreover , if in any case news of pregnancy leaks out, Rukaminj doesn't back out looking at the riches.
Basically he is paying dowry.