If you want to minimize flats I can heartily recommend anything with a kevlar belt in the tire. Not totally flat PROOF but I'd say that the kevlar belted tires I used back when commuting probably had 1/10 the flat frequency of the non kevlar tires.
Note that this is kevlar BELTED. Not kevlar BEADED. Two totally different things. Beaded refers to folding tires. Some kevlar BELTED tires might also have kevlar beads. But it's the belted you want for increased resistance to flats.
The Canondale options you mentioned both look pretty nice. But why the Adventure instead of the Treadwell 2 Remixte? A few more inches of step over for sure. But it should be a stiffer frame which will aid with maximizing the power to the wheel. Or is there enough of a position to the bars difference to prefer the Adventure?
I'm also not sure what you mean by the line below from your post. Both bikes will be able to freewheel. And I've never heard of a "clutch" for a derailleur.
Neither bike derailleurs come with a clutch but I can always tackle that issue down the road if needed.
Are you thinking of electric conversion by any chance? Or something else?