So this just cake to my mind.
Since it's clear that NishMish are acting to trigger Abir and it will not be easy since our man Abir believes in selfless love and will be happy for mishti, the trigger will be hard to find.
I think what will happen is Nannu's real girlfriend (or boyfriend if creatives are really progressive) will come into picture and Abir will see Nannu and his SO together. This will irk him as he'll consider mishti being cheated by Nannu and will try to save mishti by all means. That's when I think his love for her will really shine through.
Thoughts welcome !