"The evil Darkling cut Alina off from Maaaal!!!"
And then the first book, before the Darkling even came into the picture, starts with this:
"When we’d first started our military service a year ago, Mal had visited me almost every night. But he hadn’t come by in months."
You clearly can see how desperate Mal is to spend time with Alina when nobody is preventing him from doing so. Practically doesn't leave her side for a second. And she certainly doesn't note in an extremely bitter tone how his attention is focused on other, prettier girls.
Besides, if the Darkling "isolated poow Alina to make her feel lonely and therefore easier to manipulate", he clearly didn't do his job well: Alina kept complaining that he wa almost never in the LP, and even when he was, he was minding his busines. Alina interacted more frequently with literally anyone in the little palace than she did with the Darkling.
And then again, having Genya burn the letters didn't seem to work too well in terms of making Alina forget Mal.
But he had gotten my letters. What had he done with all of them? Had he even bothered to open them? Had he sighed with embarrassment when the fifth and the sixth and the seventh arrived? I cringed. Please write, Mal. Please don’t forget me, Mal.
Yeah, very helpful, now she is moping even harder.
But you know who motivated her to let Mal go? Baghra, apparenty.
I stood up, knocking my chair back with a clatter.
“This is a waste of time.”
“Is it? What else do you have to do with your days? Make maps? Fetch inks for some old cartographer?”
“There’s nothing wrong with being a mapmaker.”
“Of course not. And there’s nothing wrong with being a lizard either. Unless you were born to be a hawk.”
It's a shame we forgot about this thesis by book three, and Alina was made an otkazats'ya despite being born to be a Grisha.
Where are you going?” she called after me, her voice mocking. “What’s waiting for you out there?”
“Nothing!” I shouted at her. “No one!"
See? Baghra is doing a much better job at "cutting Alina off Mal, reminding her that there is no one waiting for her anywhere else. And look, it's working:
Baghra was right. I’d thought I was making such an effort, but deep down, some part of me just wanted to go home to Mal.[....] It was time to let go. That day on the Shadow Fold, Mal had saved my life, and I had saved his. Maybe that was meant to be the end of us.
And as soon as she decided to let go, she was able to summon her light.
That grief flooded through me, dissolving a knot that I hadn’t even known was there. I closed my eyes, feeling tears slide down my cheeks, and I reached out to the thing within me that I’d kept hidden for so long. I’m sorry, I whispered to it. I’m sorry I left you so long in the dark. I’m sorry, but I’m ready now.
I called and the light answered. I felt it rushing toward me from every direction, skimming over the lake, skittering over the golden domes of the Little Palace, under the door and through the walls of Baghra’s cottage. I felt it everywhere. I opened my hands and the light bloomed right through me, filling the room, illuminating the stone walls, the old tile oven, and every angle of Baghra’s strange face. It surrounded me, blazing with heat, more powerful and more pure than ever before because it was all mine. I wanted to laugh, to sing, to shout. At last, there was something that belonged wholly and completely to me.
All she had to do was to stop clinging to her shitbag of "a childhood friend" for a second, and look how much happier she is! And that's not even "an eeeevil, corrupting amplifier", it's just her magic she was born with.
Not sure at what point the plot took the wrong turn, but you can clearly see that here letting Mal go was still supposed to be a good thing.
