So I decided (author of 3 years now) to break this down piece by piece since I feel that would be the easiest so here is my attempt at explaining what I think:
1) For writer's block it depends on the resources you are using to write. An example would be if you are using in depth character outlines, chapter outlines, arc/volume outlines, etc. Some authors (including myself) might use a chapter outline for the arc/volume that is being worked on while leaving other future volumes to the imagination. For example, if I find myself not wanting to write as much after finishing my daily chapter, I will use the not used volume/arc outline to draw out more of the story so that the book itself is still being worked on but differently. The same could be said about thinking up a side character that has a bigger role instead of just "Villain's henchmen #2". So in this way you are still making progress with your book, but it also at times boosts your own morale to continue writing so that you can eventually write this awesome arc/volume or a character that becomes your favorite the more you work out his/her backstory, looks, etc.
2) I think bordum comes when you have exhausted everything about your book that you could think of. I know I get bored when I don't really want to write a scene that lasts 3-4 chapters but I know in order to move forward and get to "the good stuff", I still have to write said 3-4 chapters. I would maybe listen to some music and relax and possibly try to experiment during these times. Like if your MC and companions are in a dark cave and it is boring because all they need to do is explore it to get to where the enemy is located, you can add some comedy like one of the characters being overly scared about a skeleton or a spider. You might not be writing a comedy in that sense per se, but coming up with fresh ideas always seems to help during these types of scenarios.
3) All I can say about this, is that if a holiday is coming up or you want to go to the movies with your friends or something else, you should stockpile an extra chapter or have 2-3 stockpiled chapters on hand so that you can become distracted and pulled away, without feeling bad (other than needing to make up for it later on to keep said number of stockpiled chapters). Otherwise, you are just playing with your own willpower which is hard since if you are writing and releasing a book on WN, you have the internet.
4) This is where a chapter outline comes in whether you have 100 future chapters drawn out, or you just write 1-2 sentences when you are done with each chapter in an outline which you can reference at a later time such as the next day if you (and this is recommended) are releasing 1 chapter daily. Outlines are a double edged sword depending on how you use them though because if you spend too much time away from actually writing the book, it will take longer to get done. However, authors use outlines almost all of the time (unless you reach a point where you are comfortable to wing it which only those with more experience should explore) because they do work/help.
5) Did I mention outlines? Well, this is how to juggle multiple books whether you are writing one and see that a contest topic sparks a nerve in your brain and ideas just continue to flow in or you get an idea for a book on your own and the interest of writing it overtakes your current one. It is always good to work your creativity, but always remember that if you start a book, you should make sure it gets finished. A hiatus is fine if short term, but many authors lose current and future readers if they release too many books without completing them.
Hope this helped,
-OmniFrog