To me Mallika is never a problem, agreed she has strong relationship with Ashu that she conveniently wants to convert into marriage which never happened in the last 12 years and will never in the future too. She is like a known devil, has always tried to create problems in their life which can be sorted out easily.
Ashu and NIdhi as persons are radically different in their perspectives, life style and most importantly their personalities...she is expressive and he is too much a closed person (might be due to past life experiences).
I think for Ashu friendship is gender neutral, though he is fully aware of mallika's feelings, he never bothered to keep her away from him. He is blindfolded to an extent and only remembers the good times spent with mallika and the support she provided after BB left. Nidhi never shared the misplacing the case files, bank episode, hair dye etc with Ashu even after marriage which categorically change his thinking to an extent
Communication is the major problem among them, and he being a man always wants to prioritise Nidhi's dreams over him...even if it hurts. In child issue they both should have sat and discussed, but she randomly fixed a time frame and even said she would have cut off the time if he would have bargained. at the other end Ashu will never open up on child issue (as a matter of fact any other issue) not just it is out of his nature but it is not easy to say "I'm insecure about our relationship, with our own partner".
Even in real life, couples have issues and over the time they grown together as couple which eventually strengthens their relationship. I just wish CV's wont bring Mallika as Sanyasini to bridge misunderstandings among A-N...rather I'm happy to watch their fights and arguments.