The design itself looks good and simple. Assuming three front fans are installed and they aren't garbage. Not the single front fan option. Unless you seal the other front openings with a single front fan. I'd think you'd just get a lot of hot air recirculation.
Both designs are quite similar. I don't see how one would be inherently better than the other. It all comes down to the quality of the fans and sealing any extraneous openings on the chassis for the front intake. To prevent recirculation by the front intake fans.
Where did you get this utterly bizarre idea from? You won't get any re-circulation of any kind. The one and only time that would be a consideration, would be if you had an exhaust and an intake fan too close together.
Gamers Nexus said that the Meshify is just the Define C with a mesh from on it instead of a plastic panel. If that is true they are basically the same in that regard.Where did you get this utterly bizarre idea from? You won't get any re-circulation of any kind. The one and only time that would be a consideration, would be if you had an exhaust and an intake fan too close together.
As for why the Meshify C is better, it's because the internal layout of the case was designed from the ground up with a mesh fronted, direct airflow pattern in mind. The original P400 was designed, seemingly specifically, for a front mounted, high static pressure fan and radiator setup. The P400A is a nice enough case, but the Meshify C is still the superior product.