Hot/cold water cross-overs (please help me)
The term cross connection usually refers to the connection of a lawn sprinkler or soda machine (for example) into the supply plumbing of a business or residence. Many jurisdictions require cross connection permits and inspections of the backflow devices to ensure that backwash or foreign chemicals don't pollute the rest of the home or business' water supply plumbing.
I think you're mixing terms. You refer to the water not being hot at all, and the plumber finding a "cross connection." That sounds more symptomatic of a "CROSSED" connection...Hot and cold lines somehow reversed or blended somewhere in the plumbing system.
From my experience a backcheck device is not something that would create the problem you're describing. They're typically installed right by the device they serve, and you might have several in a building.