By the sounds of it the first thing you might be missing is how good your current role might be...

You're clearly not being considered as an equal partner, which depending on what you're considering walking away from, whether the others have already put funds and sweat equity into the project and how relatively unique your respective talents actually are (and when/if you can reasonably expect to start earning a decent salary) might be acceptable or might not. You might be able to negotiate the equity percentage but probably not that starting assumption. The job title split also sounds suspiciously like the "chairman" might not be doing as much day to day work as you and the CEO, though again in some circumstances he might be backing it heavily enough for that not to matter. (It's probably difficult for you to provide much more detail without it being identifying though)

Seems perfectly reasonable to ask prospective cofounders about their experiences with all the startups they've previously founded or invested in, and possible his answers on that (or even just the tone of how he talks about people he's worked with and backed in the past) could be revealing, particularly if, unbeknownst to him, what you're really trying to find out is why someone he invested in thinks you should steer clear.