1) They don't reveal how many applications they receive, though they tend to accept around 9-10 teams for each cohort. Since they are modelled off of YC, much of their criteria and selectiveness are similar. Geoff Ralston, an IK12 partner, is also a partner with YC.
2) I don't think there's a good, standard answer for when is the best time to apply to an incubator. It kind of depends on your background. Like YC, if you & your team are a bunch of amazing hackers & hustlers, it's possible you'll be accepted without an idea. Having an established idea with demonstrable traction is also a good thing. I would suggest applying right now, just to go through the process. Even if you aren't accepted, the application process will force you to fine-tune your business strategy.
3) Being a non-technical founder is not necessarily a show-stopper. One of the current startups, NoRedInk.com, came in as a single non-technical founder. However, this founder is a teacher, had demonstrable traction, and had already raised some money - so he entered the program with an enticing track record so far.
Thanks for your response. These make sense and I will be applying this time.