New player got a few questions

So i just started playing yesterday and after going through this subreddit, I realized that almost all of my Pokémon have 2 attacks. My first question is: Can you relearn attack’s to get rid of one attack or is it permanent? My second question would be: Do you have to place your deko to get its effects? Seems like it’s not enough room for all deko in the game and it would suck having to choose between them. And my third and final question: Do deko effects stack? If I buy a 1.5x and a 2x multiplier, do they stack or will I only get the 2x buff?