3. Is there an unseen world out there? Can we know anything about it? If yes
4. How can we know anything about an unseen world?
Here is the crux of the matter, if by unseen you mean physically undetectable rather than just not accessible to human senses.
I do not see how any knowledge of anything real can be gained without reliance on observations of some kind.
There are fields of study that look as if that is possible, such as mathematics and logic, but my impression is that those boil down to something like tautologies, which is why they do not depend on observations. (Mathematicians here are welcome to correct this, or provide a better way of expressing it, if I am astray.)
By the way, the study of history is indeed empirical. Evidence can be collected by observation as well as by experimentation.
(Please excuse my sparse posting at present. I am preoccupied with an equipment failure at work just now. Thanks to all for carrying on so well.)
Edited by Woodsy, : excuses added
The metaphysicist has no laboratory. Robert Wood