Your first point about showing screenshot to the potential costumers, seeing if they would buy it, remembers me Bill Gates selling his software when he even didn't start to build it :). I'll try to do it, good idea.
Also, as a developer who does occasional contracts to help people build out their MVPs, it would be career-suicide to steal an idea - the last thing I would want is bad word of mouth. Entrepreneurs do sometimes cluster, attend meetups, bump into each other when pitching etc.....word gets around fast in that kind of circle.
And lastly, as someone who works closely with other developers on occasion, for any given idea, half of the devs I chat to don't think the idea has legs, the other half have no inclination/time to build it themselves.
In the end, if you are hiring someone to build it for you, chat to some developers, get a feel for them, ask for a reference or two if you don't know them well (in other words, do you due diligence) and go from there.
So I think it's crucial that your co-founder be someone you know fairly well or at least someone that you may become friends with. Try asking your friends if they have friends who are programmers!
As a technical cofounder, this would be a major red flag for me if you were so secretive while building a relationship. It doesn't sound like you actually bring anything to the table. You should have connections, skills, and domain knowledge which would make it extremely difficult for someone to compete. If you don't have that, why would I want to work with you?
Also, many coders aren't that motivated to start their own companies (else the world would be 100% startups :) )