I was just wondering what would happen if someone hacked the game so they got from 69 to 70 stars while going up the infinite stairs before the last boss ? Does nothing happens while you have to reload the level ? Or does it go from infinite to finite in front of you ?

U could actually do that with a program called Star Display. Regardless, I’m pretty sure that’s what would happen. Im pretty sure that once u have 70 Stars no matter how u decide to do that or where in the game u decide to do it, it will deactivate the endless stairs and allow u to go up to the Bowser 3 stage.

it wouldn't look like anything, you'd be going up the endless stairs and as soon as you get the 70th you'd find the end.