I think Munna is Addu but Munna himself has faint memories of himself as Addu as he has been brainwashed by Shekhar Sharma and Daddaji. He has taken to the kidnappers ways and has hated his family because he somehow felt that they were happy without him. As a result he has become schizophrenic and he hates his family as he feels that Megha is happy with Mohan and Nanhi and they did not even bother to search for him. And now he hates MM even more as he feels that they very quickly gave up on him and instead had their own child, Rimjhim. (he saw them embracing on Shivratri and the episode with Rimjhim and Megha today further cemented that belief)
I really like the way in which the CVs have woven this complex character of Muddu (Munna plus Addu). Its far more interesting than Riddima who was also a schizophrenic.
An additional point: The ice cream sequence in the park was an iconic moment in Season 1 when Nanhi had her first strong feelings for her Spiderman and made Megha buy him an ice cream. So its appropriate that MN reunion in Season 2 is also over an ice cream.