He can issue an Executive Order, but those are limited in scope. The way the Constitution is set up, certain powerful acts have been delegated to Congress, such as the ability to levy taxes, declare war, and so on.
Yep.
Yep.
Congress consists of the Senate (each state gets equal say of 2 senators) and House of Representatives (number of reps based on census population). House of Reps initiates funding and revenue bills. Currently there is a Republican majority in the House.
Yes. However the composition of Congress shifted after Obamacare was passed, during midterm elections.
Unlike most issues, the right to bear arms is codified in the Constitution, and it is entrenched into a majority of the population, including some Democrats. Significant restrictions on this right would require a majority in both houses which were confident enough to restrict gun control without forfeiting their re-elections down the road.