It actually depends on one's definition of adultery, as of yet I reckon the show hasn't shown anything as such. Because adultery in my books is illegitimate intimacy, same for EMA. As of yet Nandini feels nought forget kissing and making out behind Mauli.
Yes Kunal is attracted to Nandini maybe more physically than anything else, but can we call it lust? Yes he seems to be smitten but as of yet he is controlling whatever hormones that drive the male gender. So for me he is an adult with issues; whether he is wrong or right that is up to each individual? But the fact that he is fighting, shows he is a decent guy. Do I like it? NO of course I don't but it seems genuine, the actor playing Kunal is doing a fantastic job.
You mentioned the biggest problem you have is the music and the general romanticising of something that is wrong, more specifically the projection of EMA in a positive light. However I see it differently, it isn't stemmed into any light neither positive nor negative. It is simply projected differently then what we are accustomed too. The 3rd wheel isn't always the evil tart, or the psycho lover. With Silsila they are trying to show that.
I understand it isn't everyone's cup of tea, heck even I am watching it for the sake of DD however I do love how they are showcasing it, the story telling is really good.
And yes Bepannah is also along the same line so it may seem Colors is treading on the EMA path. But each to their own, while Silsila deals with human complications, Bepannah moans over the death of their cheated partners.
I understand shows like this come with complications, however Shows/Soaps are a means of entertainment and if you feel it derives an ill thought then it's best to ignore. Take it with a pinch of salt, because it is the reality whether they show it on TV or not, it is happening. I see it as Maleficent, the wicked witch was hated but then we saw her POV, here they are trying to show the POV of someone else that isn't the wife. I understand the repercussions EMA has on a woman, and believe me I so feel for Mauli but Nandini isn't to blame not yet anyway and from the promos, what I understood is both Kunal and Nandini do eventually fall for one another but they let it go, so isn't that saying something.
Anyway I reckon you should give it chance before forming of any conclusion or simply ignore it
That's it for now, SOS it is a bit too long lol