Have anyone noticed that the reason why some persons post go off the topic is because they don't read the entire op writing before they reply to a thread, some persons respond to a thread based on the caution of the topic and that is why we meet up different opinions or suggestions that is not in lines what the OP points of view.
Secondly, when someone have written what you want to write in same thread you have to give off, instead of making a pointless point, I know very well that many users in forum make off topic because what I outlined.
I got your first point but I did not get your second one, speaking of the first one, you are right, when we see a new post it has most likely not been checked by the moderator or some person who used to report it to the moderator, so you saw a new post and made reply to it, and there is a chance that mod will move that thread to off-topic section if its not relevant, otherwise to the relevant section. I have faced this so many times, as I mostly reply on new topics and some of posts end up in sections where I don't want them to be. But that's ok.
Besides realizing the context of the post of OP, we can give some time to the post, maybe a day or when 1 or 2 pages already created we can make post on it without any worry, as 99% chance is, that topic is not going to be moved now. But the best practise is not doubt to realize the post contents, I have done this too many times and also report the post to the mods so it already end up at relevant section, so other's efforts would be saved. But still, we write what we love or like to write. So overall, no loss even if post is not counted or considered by other members as member don't visit some section so you get lesser response and replies.