A Carroll EMC employee says that the 200 amp main breaker on the breaker box protects either phase [120 v] at 200 amps. I conclude, therefore, that the total available amperage at 120v is 400, assuming no 240v circuits are wired. Is my assumption correct, and/or is the CEMC employee correct?

I had always assumed that the total amperage was 200 and that the main breaker would throw if total amperage reached some high percentage of 200.

This can be confusing. A 200A residential service is 200A @ 240v. You can say this is equivalent to 400A @ 120v, but this is not really accurate.

Bottom line is this service will deliver 48,000 watts worth of power.

You are correct in that the main will trip if either leg of the service (or both) reach somewhere well over 200A. Breaker do not trip as soon as they exceed their rating. There is quite a trip curve.