Sorry for the late reply, but the answer to all of those is pretty much "If you believe it." When you get into multiple planes/dimensions you start entering an area alongside other things like religion. They can't be proven nor dis-proven. However, for the first one while your mind is in a dream it is also still semi-aware of the outside world. For example if your brain was focusing solely on the dream nothing would wake you up, and we'd die out as a species.

For the second one, your can dream anything you want, but whether you believe it to be real or not is subjective.

Dreaming can and is considered by many to be a different reality, simply because of how it operates and how we perceive it. When you start talking about things like this the word "reality" doesn't really hold any meaning as reality is just your perceptions as you interpret them, hence we all have a different reality from our point of views.

Ideas are like a virus; They spread rapidly and are difficult to get rid of. They can affect how you act and what you do. Whether or not they not only exist but grow in alternate dimensions are unknown, as previously stated you are stepping into a "grey area". However, if you add action to that idea then they become a part of our reality we all believe to be the true reality. But if you keep them in your mind then they aren't considered "real" by 99% of the world. As for them being "dreams/concepts molded together as on solid thought creating a world induced through 'drugs'", I can't really comment on here since I don't completely understand the question.

I hope this helps

EDIT: Also, if you come across someone who outright discards your ideas/beliefs then that person is just an arrogant, close-minded person. Don't let them control what you believe since they obviously won't let you or anyone else even comment on what they believe unless it's in complete agreement.