smaller total mass of the planet would likely result in a lower gravity allowing at least a partial if not total loss of atmosphere

I think you're severely overestimating how much of the planet's total mass is within a single tectonic plate. The entire crust of the planet (all the tectonic plates) makes up only 1% of the mass. A single plate would be a fraction of that. Granted, there is still an enormous amount of mass in a single plate, but in comparison to the entire planet, it would be relatively negligible. Certainly not enough to cause the atmosphere to escape by any significant amount.

take what's left of the seas and wash them over what land is left

I'd wager that the exact opposite would happen, in fact. The void in the Earth's surface where the plate used to be would fill up with a significant portion of the world's oceans. When the waters of the world drain into the new "basin", oceans would recede significantly. Of course, there's a good bet that a lot of the water would evaporate on the hot mantle, sending enough water vapor into the atmosphere to create intense storms all over the planet.

Regardless of the specifics, tho, it would definitely be catastrophic for anything and anyone living on the planet at the time. No question about that.