I'm stressing about the weight of the 700x28 Marathon Plus tires in my cart at almost 2 pounds apiece...

I agree with Lex that the rider's weight shouldn't be a concern. The load will influence ideal inflation pressure, but I haven't heard of any tires with weight limits or any that are particularly well suited for heavier riders.

Generally a tire designed for touring will be more durable than one designed for racing.

I guess I need Schwalbe Marathon Supremes, but can't find a source. Schwalbe is sold out of 28s.