This should help you change the material via blueprints. and if you want to change the material at a certain time you can do it with a trigger (if you want it to change at a certain point in the level) or use the delay function in blueprints.
For your questions about making the side scroller features for your game there are many tutorials on youtube that do an amazing job explaining this. I am not sure if Unreal Engine’s channel has a playlist yet but I did find a good one Let me know if none of these are what you are wanting and I can do a better job at answering your questions.