"If Bob Dornan does enter the race, I could see Brewer winning."
In California, special elections have different rules than regular elections. All candidates run on a single ballot, rather than in seperate primaries, and the first place finisher of each party becomes the nominee. Then a runoff is held, unless one person wins an outright majority of the vote.
For example, if Marilyn Brewer wins 32%, John Campbell wins 31%, the 'Rat wins 25%, and Bob Dornan wins 21% as the American Independent, then there will be a special election between Brewer (R), Dornan (AI) and the 'Rat.
Such a senario is very feasible.