I don't share your feelings that Taylan is clearly out.
If the DNA hasn't cleared him, it's almost impossible to see how it can't be him.
Perhaps Taylan pursued her by phone, IM, and email, the whole time, asking for dates and arranging hook-ups. We don't know. Maybe she was fending him off the whole time she was there, and it was her who wanted the occasional hook-ups while he was gunning for more.
Surely you are aware that lonely people fall in love with others online. They are willing to send their life savings, get married, and do crazy things for what they imagine to be love. This love may not have followed the pattern you find typical. That doesn't mean it didn't happen.
All your speculation as to his feelings. I could just as easily speculate the opposite: Taylan pined for SS and the secrecy of their hook-ups only intoxicated him further. The more she tried to keep their relationship occasional, the more he read it as heating up into something enduring. After he kills her, when he hears from the husband, he definitely wants no part of the trouble. If he's so innocent, why didn't he act like a typical player Ammer and be cool and do what he could to help track this missing woman down? Instead he went into CYA mode as a guilty man would.
There's no indication of WHAT SS meant to him, period. The paltry evidence can easily be interpreted to suggest his guilt. Maybe she wanted to see him because the night before, they had sex, and she had been swayed by his romance and talked about building a life with him. The next morning, she realizes this is madness, and wants to see him because she had felt something for him and wants to let him down easily. See how easy it is?
There are multiple explanations of the evidence possible.
My feeling is that if he hasn't been cleared by DNA evidence, Taylan is candidate number one.