If I forward a message from A to B. B replies to me and is added to the thread. I now reply to A's original message. Will A be able to see the forwarded message and reply from B?
If you reply to the original message, the one from A, no he/she won’t be able to see that you forwarded the message before replying to it. The thing to pay attention to is to reply to the very first message—the one that you received from A—and not to the one that you received from B as a reply to the forwarded one. Gmail is intelligent (or sneaky, depending on your point of view) and groups all these emails into conversations and when there are more persons and reply/forward interactions involved, you need to pay extra attention to exactly what message you are replying to/forwarding.
Think it this way: the original message is the point of origin. By replying to it, you are creating a branch. By forwarding it, you are creating a different branch. So be careful where you are situated when you want to do something.