Generally yes, unless stated otherwise.
Basically assume that person always was that gender and had that name.
There are some edge cases with people like actors, where things like their work is footnoted with something like "credited as XYZ". although some people are feeling even that is still bad. (I don't have a skin in the game, this is just what I know from trans people around me)
Yes but Elliot page wasn't in this movie. Ellen Page was. If he makes a movie now of course it's Elliott page but I think it's weird to go back in time and erase things.
Name changes don't even have to have a transgender component to be in perpetuity. Plenty of people change names due to marriage or they just hate their name. That is Elliot Page. Plenty of actors with names that don't necessarily match their gender. If the person wasn't trans and had changed their name to Isabella, you'd just call the person Isabella no matter when a photo of them was taken.