Engine accelerates above throttle rpm

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The opposing torque comes usually from the wheels (tire friction), translating engine torque into car forward motion. There is no such thing as throttle_rpm.

It cant be from tire friction, throttle(or gas) => rpm relationship works on neutral gear too. According to torque curve if engine rpm is between idle and limit, then engine produces some torque. If it always have torque on neutral, then how can engine rotate at constant rpm (when gas pedal is in constant position)?

It depends on how complex you want your model to be.

The opposing torque in the neutral case is engine friction. You choose the throttle position so that combustion torque + friction torque = 0, which results in constant rpm. This throttle position depends on combustion and friction torque curves and your target rpm.

But as soon as you have an additional load (tires) your rpm will drop and you will have to adjust throttle to get it back, keep it constant.