So, Bob. I'm going to call you Bob. You sound like a Bob.
Bob, you suffer from the same strain of arrogance as many academics, especially those with a bit of British education in them. You think we're wrong - fine. You're bloody damn sure we're wrong - fine. You're convinced that, having peer-reviewed the evidence available, we're completely wrong about everything ever - and that's also fine. But what you clearly don't understand is the damage you're doing to your own cause.
First of all: Bob, if you want to invite someone to debate you, no matter how insane you may perceive them as, it is not a good idea to write multiple paragraphs about how all you intend to do is take the Mickey. Combining this with a half-hearted sentence about how you mean no harm is extremely unconvincing. Now, if you're doing this to deliberately sabotage your "efforts" in setting up the event - fair enough. I understand that you don't want to be doing this, but perhaps it'd be better to show some integrity and just tell your colleagues that you're unwilling?
Secondly: you say that because you're a non-profit, you ask that we cover our own travel costs - for an event which, as you clearly stated, you only expect to benefit you (as a source of cheap laughs). We're also a non-profit. In fact, we have precisely no sources of income. So, Bob, I'd like to propose a counter-offer: you pay for my travel costs (after all, it's your event, and you do charge for admission to most events by the looks of it), and I'll bring you a couple beers from our local brewery.
Thirdly: you repeatedly said that you would want to vet potential "candidates" on their scientific literacy. You have made no attempt at making a similar offer back, and judging by what you've described so far, your understanding of the Flat Earth Theory is rather poor. Once again, you make it clear that you're interested in a very one-sided joke, and not a discussion.
These attitudes, as I've said many times before, greatly contribute to our growth. So, by all means, please continue to act like this in public. It draws great amounts of attention to us and really helps us develop a movement that now spans the plane. However, as far as your "event" goes, I'd like to recommend following common sense and being sincere with your colleagues. You don't have what it takes to respectfully recruit someone who disagrees with you. If they want to get serious about this, they need to find someone more qualified than yourself. Perhaps someone with his head not stuck so firmly in the stars, and one who can actually put aside differences for a moment.
Alternatively, you can go the "welp, I've tried, these lunatics couldn't be convinced by my perfectly rational offer" route, and skip straight to that local brewery of yours.