No advice at all I'm afraid, and I'm so sorry for you all.
With regards to your DD, you're right when you say that she is grieving - she's also angry, confused, scared - lost even.... she is a 15 yr old ball of emotional turmoil, who has loads of questions that will now never be answered, loads of feelings that scare her, and that she can never articulate.
Regardless of her stepdad's love, she still has to live with the fact that she was categorically rejected by her natural father, and now will never be reconciled with him.
Despite her Stepdad's love, I'm sure (almost certain) that in the back of DD's mind, was the fantasy idea, that her father would turn up one day & 'claim' her, that there were extenuating reasons behind his abandonment of her, and that he longed to be with her - like she did with him.
To be honest she probably blames her stepdad on some level (though I'm sure that deep down she knows it's not the case), and believes he 'stole' her away from her real dad - this is more likely to be in her head now, as she comes to terms with a total loss, and an unrectifiable postion.
I agree that her stepdad's behaviour may be worsening things at the moment, as she may see him as being insincere, and herself as unlovable ("if my real dad didn't love me, then how could SD). I wouldn't necessarily change things though, as he has been her 'dad' for as long as she can remember, so I think this is a reaction to how she feels.
I'd suggest counselling, she sounds like the depth of her feelings, and the insecure feelings it has dragged up, are unsettling & overwhelming her.
It's also possible that she's deliberately pushing stepdad away, as she's scared he may leave - facing mortality can be a shock.
I'm sure your daughter will realise how much her stepdad loves her, but she's rebelling (like all teenage girls), and right now is trying to figure out where she belongs.