I need to attend a conference in France for about 10 days but Geneva is my entry port. I am unable to get the visa through France as they don't have any appointments until late Sept. Can I apply through the Swiss embassy? I can try to stay 2-3 days in Switzerland if that helps.
No, Switzerland is supposed to reject the application and invite you to apply to the French consulate. Even staying the same length of time in both countries might not be enough, as attending the conference is clearly the main reason for your trip, which makes France the main destination.
In practice, you could still try. In principle, they should not refuse the visa but merely decline to process the application, which means that you should get back the visa fee and the documents you submitted and there would be no record of a negative decision. So the damage should be limited and you don't risk much by trying. But it's still unlikely to work (and it does take some time, obviously).