> A breaker will trip on a peak current. It's not much of a 30A breaker if it lets 42.5A past now, is it?

A thermal magnetic breaker trips at different rates.

Magnetic circuit trips the breaker when a short or near short is experienced. They are rated to trip at 2 to 5 times the rated current. Whether 2, 3, 4 or 5 times depends on the type of breaker.

The thermal circuit will trip the breaker when enough heat is generated due to the current flow. Since is it heat related the current is sensed as RMS.

This is a safety and a local code issue. Please do not suggest solutions where you don't know the complete situation nor the local code requirements.