Seats at the Monte Rio Community Center were full Thursday night for what residents thought was the final step before county supervisors forced them into an unpopular and expensive plan to replace their septic systems.  Clarity only came late in the meeting, when Deputy County Administrator Barbara Lee attempted to calm frustrated residents. Until then, the prevailing assumption was the Sonoma County board of supervisors would decide in January whether every household in Monte Rio and Villa Grande had to connect to a new sewer line or create community leach fields, all at a cost of tens of thousands of dollars per home.