I agree with your summary except for the bolded parts. I so far don't really see anything that says she was bored with her husband or found him dull. Traveling without him doesn't have to mean there was a problem, just that she had her own life and interests. And I'm still in the camp of buying her IG posts that there was a problem with her college enrollment that she had to attend to in person. She didn't change her plans directly before she left - her post about leaving early came somewhere around the Amsterdam trip, and so she must have changed that earlier. Mix in some homesickness for her boys. I get that....
I saw some posts Sarai made to others in NY about getting together for photo walks. I got the feeling they'd done that before a few times (she says "another photo walk" in her posts about NY). So this trip was probably the international extension of the photo walks.
I personally think most people are good, non-murderers, non-rapists and so the overwhelming majority of the time your interactions would be positive for getting together. Sarai sounded like she had really good interactions with IG people. Her A'dam "guide" seemed like a smart and fun person, too. I can understand how she felt confident about something she had done before and enjoyed.
Okay here's my thing about Taylan.
Didn't she leave the house THINKING she was going to meet with Taylan? She asked to meet with him, he said he'd meet with her, she went to the Bridge intending (I'm assuming) she would meet with him. So why would she have met with a different IG'er instead? How would she have changed her plans so quickly that when Taylan didn't show up and she got tired of waiting for him, she was able to contact yet another IG'er and arrange to meet with him instead? And how would she have communicated? She didn't have her Turkish cell with her (by all reports) and so she'd have to use wifi, and as our "locals" have said, that's not necessarily easy to come by on the fly. She would have to have stopped somewhere, asked to use the internet... that sounds like something someone would have remembered, a waiter, etc.
So that's why I think it was either Taylan or someone she met while walking around... because she left the house believing she would probably meet up with him and so I can't see how she'd have made other plans with someone else at the same time.