Needed PCB surface to cool a SOIC-8 EP package
This is actually a pretty deep question. Fortunately, there is an extensive layer of literature on this subject. You are basically on right track, but, for some reason, didn't get to the right articles. 

Yes, if the pad is soldered down, you can assume the θCS as zero. 

Regarding vias, typical thermal resistance of via over a 1.6mm FR4 PCB is 130 to 250 °C/W, depending on stack-up and via size. So you would need few of them to have any effect. Or you can make one 2-mm diameter via, and fill it with solder. There are plenty of calculators on this subject, Google for "Thermal resistance of via calculator".

All details are perfectly explained with formulas and practical examples in this Application Note AN-2020.

Final results will depend on details of ambient conditions, whether the board is vertically oriented or horizontally, are there obstacles to natural convective airflow, or there might be some forced ventilation around. A thermal image of PCB will help tremendously to evaluate the board thermal condition, and, if necessary, design corrections should be made. 

But for a 1W dissipation and 3x3mm thermal pad soldered to 1.5 oz PCB, I wouldn't worry much, given TJ = 150 °C.