Since Hagia Sophia is currently a mosque it is open every day of the week and with no need for entrance tickets .. there may be hours in the day to avoid such as prayer times- that’s all .. check with your guide

Yes, contact viator ahead of time. Our guide was Tayfun. 8886519785 is the international number. We saw all of those amazing places with him.

We did pay for transportation thru thr agency, bur most of the tour was walking so transport was just good for hotel pickup and drop off. Service was great, but tram or subway will do the job fine

We also paid the guide at the end of the day and didn’t have to queue. The guide normally purchases the tickets beforehand.

It's a personal guided tour so they will tailor it to your needs and requirements and availability of the sites. If you want to see specific things and have clear interests, make sure you tell them ahead of time. At some point our tour went a bit rogue and improvised, because we didn't have a clear picture of what we wanted. So we ended up in a carpet production (which was interesting and totally acceptable) which devolved rapidly into a high pressure sales situation for USD 20'000 kilim silk carpets (which was not acceptable). We managed to get out. At the end of the tour, I was asked to write a tripadviser recommendation and the guide was constantly looking over my shoulder as I wrote it.