Yup, that's the premise. It's just an annoying thought experiment. Your actions physically can't change the past, but somehow they still do, because the past was decided based on a perfect prediction of your actions. I was just playing devil's advocate. I agree with your answer 100%.

"Now" is the moment where you decide whether to pull the lever. As is conventional in trolley problems, this moment can last anywhere from 2 seconds to hundreds of years :)