Need more information. It doesn't seem rationale that they are protecting the same circuit unless you are substituting one for the other for a test. A unit like an air conditioner may have multiple breakers and a higher value breaker may trip because the load it is connected to is drawing current in excess of the rating of breaker.
It would be higher
A 15 amp breaker will trip at 15 amps at an ambient temperature of 104 degree F. If the ambient temperature is higher the breaker will trip before 15 amps and if the ambient temperature is lower the breaker will trip after 15 amps. I would suspect the circuit is overloaded. But, you can change the breaker and see what happens. Just swap it with another one.
It would be higher
Lower.
a plateau would be higher
The voltage before the breaker is from the distribution panel's buss bars. This voltage is controlled by the panel's main breaker. To have no voltage on the terminal of the breaker means that either the breaker is shut off or it has gone into a trip position. If it has tripped push the handle of the breaker to the off position and then to the on position. This should reset the breaker. If the voltage is not present at the output terminal of the breaker after resetting it, then replace the breaker as it has a fault in it.
lower
Higher.
lower
faster in higher altitiudes
Higher
In the Northern Hemisphere the curve would be higher, in the Southern Hemisphere it would be lower.