Sure, so long as she keeps it to herself!

Absolutely. There is a real possibility that we have already had an atheist president. Openly atheist might take time , but its dependent on the candidate. I doubt anyone will ever run on a pure platform of being atheist, so it would likely be a popular progressive that appears to the left but is likable enough to win over some moderate republicans.

the basic answer to that question is "no." Americans still have a majority distrust of atheists and agnostics and I doubt they would vote for a person who will not take the oath of office on a Bible (though the Constitution does not require either a Bible nor the new president saying "So help me God." )

Religion has become an intricate part of the American culture that it will be hard to separate it from the political aspects of our representative democracy. I do not see a change in this position for at least the next 50 years. (That is if we don't get into a nuclear war with North Korea first.)

Obama was not a believer but had to act the part! Trump is not religious either!

I don't see why not.

You mean besides the current one?