Answer to Question 1
3
at this point in time the three fundamental forces are gravity, the electroweak, and the strong force. The electromagnetic and weak forces were proven to be facets of the same force in the 1970's. GUT's strive to further unify the three forces into 2 or even just 1.
Answer to Question 2
Using the ideal gas law PV = nRT, you can expect a pressure drop to P/2.