Yes, a president can resign. Richard Nixon did this, when he thought he was about to be impeached. Rather than wait to see what would happen, he resigned from being president in August 1974. I enclose a link that will tell you more about that event.
This is a complex question to answer, one that the Supreme Court has argued over a number of times. The answer is that the Supreme Court has interpreted the Constitution as vesting some power of oversight of the executive branch in the President, which includes the power in some cases to remove the officers from office. This power does not seem to be absolute.
For instance, in one famous case the Court struck down a law which forbid the president from removing a postmaster general without the consent of Congress.
In other cases however, the Court has refused to strike down laws that limit the President's power to remove officers, such as by requiring that he have "good cause" before he removes them.
Some cases suggest that the President should always have the power to remove "principle" (i.e., high ranking) officers, but that Congress can limit his power to remove inferior officers. However, some of the Justices, particularly Justice Scalia, disagree with this view, and believe that separation of powers requires that the president always have the power to remove officers of the executive branch.
yes
The military does not have the athority to remove the president from office.
The Senate in the United States has the power to impeach the president and remove him from office. The president does have to have a trial prior to impeachment.
The Senate in the United States has the power to impeach the president and remove him from office. The president does have to have a trial prior to impeachment.
johnson
No
yes
Remove him/her from office.
Appoints someone to the job
Tenure of Office Act
Treasure of office act
The answer is Andrew Johnson
The Heads of Cabinet departments