Hello and welcome to the forum,
That's a lot of questions and it would take a long time to answer them all. Of course, you can create your own combat system. You have to use methods on the player/event.
For example, to move a monster towards the character, you can use the moveTo method
https://docs.rpgjs.dev/api/MoveManager.html#move-to
Feel free to have a look at the examples: https://playground.rpgjs.dev