Carry out should generally be used for carrying out actions - instead should be used when the action the player types shouldn’t actually happen and rather something else should. It’s very easy for new authors to fall into the habit of using instead rules for everything, though, since it doesn’t require you to dig into the complicated action-processing system. But it’s good to resist that temptation if you can - here’s a good thread on why:
Oh, and as to the benefit of multiple rules vs one with nested if statements - either can work, depending on your stylistic preference. It can often be a little easier to see how things interlock with one rule, but it eventually can get unwieldy, and the ability to manually shift what order rules fire in can sometimes be useful. So sorry, that’s not very helpful!