In practice in the U.S. the early part of a PhD program can be very similar to a master's. It's relatively common for people who leave PhD programs to walk away with a master's degree they didn't intend to get. It's called "mastering out."
Yes you can also leave with an MPhil in the UK if it goes wrong.
I though dissertation referred to the final written document you had to produce to get your PhD. Most people I know with PhDs stared writing that during the last year of the PhD program.
When you're writing papers during your PhD you're basically writing chapters of your dissertation.