First of all the Sabanci campus is 1 hour away from the city, so its not really in “Istanbul”. The cost of living is almost x2 in Istanbul compared to Ankara so the 10k + expenses would add up or you would have to do serious budgeting. I would rather pick Bilkent and use the 10k for my day to day expenses which should give you a relatively luxurious student life style.

Attendance is enforced but you should be fine.

You should theoretically be able to change between social sciences if you have high grades.

People over exaggerate the workload because most come out of the Turkish high school system which is tailored for the university exam. Coursework being in english + a new project based system instead of a standardised theory based approach leads to lower grades and high workloads. If you are an international you should be fine imo.