1. I'm not sure if I can answer this correctly, but consider a thread as having a life of its own.
The moment in time you first posted or found a post, is gone. Others have posted and the thread will never be the same as your first look. I've worked on a reply on a new thread, and when I posted it a few minutes later, my reply was way down the page as the thread might have had 50 other people posting at the same time I did.
If you are having trouble finding something on a thread that's gotten past a few posts, be sure to click the 'View All' option at the botton of the OP or if you're looking at the 'Latest Threads' page, click it there if it has gotten big enough.
That will show everything. If you are looking for a reply to your own post, you can find it on 'My Posts' and find where you were.
Hope that helps, you may have known all of that already.