I disagree because he has had other reassurances about Madhu for him to trust her. So she has passed other tests which were not even pre-planned like this one was. When he was shot, she could have sat back and allowed him to die and be a rich widow, instead she fought for his life by praying, giving him blood and caring for him afterwards, mukunds case he wanted to bribe her with money she refused the money when he ruined her reputation she could have have sold her story to the press or joined big boss or another reality show increasing her fame and fortune but she didn't do that, during karva chauth vrath she fasted though she was in physical pain, she had a huge thirst from the kamaam he knows about that also he found out that to fulfill the rituals she at the sargi from the bin, the KC was again to prolong his life so they could be together in their married life for longer. Those were the reassurances she has done already, by listening to Dipali, who was the 3rd person to betray his trust his proving that his insecurities will never be assuaged, she will prove herself momentarily he will be satisfied and the following week Dipali again comes to whisper something else and again he mistrusts Madhu and she has to go through another hurdle to satisfy him, a loop of mistrust. Also RK knows the value Madhu has on family, the test included leaving her family and everything she has shown him upto now has shown her to be a family oriented compassionate person.
All the above i've answered from RK's perspective as your post was talking about RK having a reason.