He did not return for Abir. He returned to have a sense of purpose in his life because his existence is basically defined by his son. Irrespective of whether he is guilty or innocent, Mehul simply lacks character.
You can have a shit load of life philosophies but ultimately how you conduct your life is what really defines you and creates your identity as a person. Mehul might not have done any fraud but he is such a coward and such an asshole that despite of him running away, he feels he is entitled to getting to know his sons and be with them. If your family mattered all so much to you then why didn't you fight tooth and nail to prove your innocence. And when you were never really able to fight to be there for your children when they were kids, what gives you the right to think that you should be with them now when they are adults.
I particularly feel enraged whenever he tries to pose as a father to Kunal and expects that Kunal accepts him and embraces him simply because he happens to be his biological father. Like puhleaseeee, you have no right whatsoever to expect or demand that love, affection or respect from Kunal when you were never really able to there to fulfill the fatherly duties for him.
I am truly glad that Meenakshi did not have to spend her life being with such a spineless man who basically did nothing to earn the love and respect of relationships he poses to be so entitled to.