But as for when to press A, i just do it when mario lands back on the wooden block, it wont work it you do it while mario is growing. You can also hold right the whole time if you want, don't have to release it like in the video

I didnt record that video. Was just something i found while i was learning the game so im not really sure

Should also point out you can ignore the random up dpad presses in the input display in that video. Also you may not want to do the strat where you jump over the koopa at the start

Someone else may be able to give you a better answer, but basically for 1-3 you can do the strat where you dont land on the goomba like in this video (which has input display aswell) .That strat seemed easier to me and the input display helps alot for learning it. As for the white block you just have to be holding down on the d-pad to go behind the scenery you don't have to actually crouch. Hope that helps