Does anyone make a mixte frame that's not steel? I don't know if I've ever seen one. I think your options are pretty limited. I haven't seen the Origin8 frame in person, but I'd be pretty shocked if you could fit 40c tires and fenders into the caliper brakes you'd have to put on there - I imagine they take a medium reach brake at most, and you'd probably be limited to 28c with fenders. Soma makes a mixte frame that I've looked at at the LBS which is pretty nice, and looks like you can squeeze 32c tires with fenders if you're comfortable with small clearances. The Soma takes caliper brakes. And yes, you can use road levers with any of the brake options on any of these frames.

You could get bigger tires on by using 650B wheels and a longer reach brake, though you'd want to do some research and measurements to make sure whichever frame can fit what you want to put on.

The Intec's dropouts, indeed, are standard horizontal dropouts. Some don't like them, because it takes about 10 more seconds to get your wheel aligned and straight after you've fixed a flat - I think they're great, and give you a lot of flexibility on what drivetrain you'll use (derailer geared, single speed/fixed, internally geared). I also can't think of any other new production mixte frame that has cantilever posts - and would be surprised, again, if such a thing existed.