It is not the idea to bother them, but whenever you can you should consider participating in the threads they use to write.
I don't think a user should participate in any discussion just cause merit sources or any other user at all is in it, post in discussions cause you have something to contribute to it and you're comfortable doing so, and mind you that a merit source or other users mustn't necessarily join in a discussion before they give out merits, so just post wherever you wish and if the posts are good, then they'll prolly be merited.
4. Should I reply to every one who reply to my post? ( 4th number add now because I think this is compulsory question)
No, it can make your thread look disorganized, and there's no need to do so if you do not have anything useful or necessary (mind you that 'thank you' isn't necessary) to add to what the user posted, and then if you want to react to two or more replies, you can wait it out and through quotes include all of it in one post.