I assume by "impeached" you mean "removed from office"? Impeachment, done by the House, is the equivalent of an indictment. Then it goes to Senate, which would have to convict Trump with 2/3 majority for him to actually be removed from office.
That said, even if Trump is removed from office or resigns, unless Pence is removed also a Republican will be in the White House through 2020. Inauguration is not until January 20th 2021.
Maybe, maybe not. I think Pence would run a lackluster campaign and would probably be easier to beat than Trump.
But at the same time, it would be a feel good moment to see him impeached and held accountable for something finally.
Again, he'd also have to be removed. Otherwise, he can claim that he was vindicated via acquittal by the Senate. Impeaching and not removing would probably be the worst outcome for Dems as far as electoral chances go. Which is why Nancy Pelosi was reluctant to call for impeachment.