You're citing Einstein's Theory of General Relativity while ignoring the most basic concept of relativity. Light is bent in the influence of gravity only because all of spacetime is bent by the influence of gravity. It's not just light that is bent, space itself is bent, distances are bent, mass is altered, time itself slows down. That's how you can bend light and still have it travel the exact same speed - because
everything is bent there.
Yes, the concept is famously difficult to understand. In an introductory physics course they will spend a couple weeks focusing on the results of Einstein's 1905 theory of Special Relativity (the changes in physical law that occur when observing objects from frames of reference moving at different relative speeds), and only then would move on to Einstein's 1916 theory of General Relativity afterwards. It's not going to be adequately explained with no context on an internet message board - take a course.
That's completely false - the speed of light has been measured many different ways, including in "one direction" for hundreds of years.
Speed of light - Wikipedia