You start researching a novel when you begin to read. That isn't a crazy answer. I became interested in the Napoleonic Wars when I was a child, and I have been reading about them ever since. Research is a lifelong occupation, but obviously, for any specific book, you begin dedicated reading and research a few months before. So, I've spent a lifetime reading about mediaeval warfare, but the detailed research for Agincourt (the book I'm writing now) probably began about a year before I started writing. And the research goes on . . . right now I'm writing the last chapters of the book and have eight other books open on the desk.
None, really! You send them the finished book, and of course a good publisher will suggest changes (which might or might not be good suggestions). Those changes are the editorial process . . . . some editors will want vast changes (why not set the book in the 20th century instead of the 15th), but mostly they are small details (in chapter one you say it's April, in chapter 2 you say it's summertime). The biggest involvement publishers have in writing is to encourage it with money.
The advantages: a wonderful commute, a very elastic dress-code, the joy of telling stories for a living. The disadvantages? None that I can think of! I love it!