I strongly agree with this analysis.
Here he is, an unemployed, but educated Turkish man who connects worldwide with people over photography. I'll guess 75% of the people on those IG-type sites are male and few appear to be single "available" females. Then all of a sudden pretty Sarai Sierra responds to his sly flattery and not only that---- she is coming to Turkey to see him! How much more ego-stroking can it be? Regardless of her words, it is easy to imply intent for a sexual liason in this development. She is not coming with family, or a photo club, or a tour. She is not stopping by for a day or two, but three weeks. Huge opportunity for him? In his mind, what is going to happen? Are they going to have a mad affair? Is she going to take him to the United States?
I think this must have started off akwardly when she stays in the least expensive place in a kind of crummy neighborhood and she is not the libertine he thought she might be. However, never discount exotic situations for the promotion of romances. I think she did not disclose her marriage or children early on. Maybe not until the last day. Would that be enough to turn someone into a furious angry rage- that they had been "deceived"- in their mind- and not going to be taken to the US, etc. Maybe SS had taken a fancy to a different IG contact and TK felt threatened. One of the other 4 internet contacts LE is still interested in. It's a thought.
TK is still the only person we know of that had contact with SS her last day. The turkish cellphone must have communications on it that LE is analyzing. It is difficult for me to believe TK only saw SS twice in that entire trip, and the first time was when she had been in Istanbul for 5 days. That just doesn't add up at all.
My other contention: SS was going to meet someone she knew and liked that last day- the different bag, the extra pair of shoes for a social occasion, the jewelry.
With the void of other suspects, TK is still the prime one. However, the other internet contacts not known to us are also in the same highly suspicious group.