Mishti can choose ishan now and he can be the perfect dad but if Kunal becomes desperate to father mishti and to meet her and shower her with love and mauli rejects him and hides this fact from mishti then it is bound to be revealed eventually and may cause issues for mauli and her relationship with mishti. The yearning for our biological parents is strong and automatic. Sometimes adopted kids who are raised in loving homes eventually grow up and wish to meet their biological patents. I've seen these stories in real life. It happens quite often in the west. The yearning to see them, know their names , touch them, hug them...it's powerful... Therefore if your child's biological parent pulls his shit together and is dying to be a part of the kids life then you should let him. Not everyone is lucky to have living parents. Might as well bond before time is up. If mauli refuses to let Kunal see mishti then it will have nothing to do with mishti at this point but will only be a continuation of the saga between her and kunal which is now 6 years old...and willturn into a revenge saga involving mishti.
Moreover one day mauli and ishan may have more kids. Those kids will receive ishans love and mishti will then become the step child. If she has a living father who wants her at that point, will mauli still keep him away? Would that be justified? When the guy seems capable of parental role and is raising one kid then let your child meet him for herself...it's abt her bond now with her dad. Mishti meeting Kunal doesn't in any way reflect upon the endgame but definitely may shape mishti own life...every girl deserves to be her dad's princess and if the dead beat dad finally sees the light then let him in.