driving a synchronous motor as a generator
You don’t really drive the motor at 100 kW. You drive it with a prime mover that is capable of supplying mechanical power up to some limit. If the mechanical load is still connected to the shaft, the prime mover will be loaded to 100 kW. If the motor is to be connected to the electrical system, the prime mover speed and the motor/generator excitation would need to be adjusted so that it is synchronized with the system before connection. That can be done in a way that prevents the motor/generator from drawing power from or supplying power to the system.
If you wish to have the motor act as a generator, you can adjust the excitation to supply up to 100 kW. If the demand on your side of the meter is not more than 100 kW, you can adjust your generator to supply all of that demand and nothing would be drawn from the grid. The prime mover would need to supply 200 kW plus an additional 5% or so for generator losses.
If the motor is rated 100 kW, then any more than a 100 kW load as a generator would overload it. It would overheat, but more importantly, it would lose synchronization with the grid and potentially cause a lot of equipment failures. If your own loads use less that 100 kW you could export energy to the grid. If both the motor was rated higher than 100 kW and the prime mover was rated more than 200 kW, you could export energy with your stated 200 kW total load.