Using a motor as a brake
What will happen if the resistance is lower than that? The vehicle is
moving and is generating plenty of mechanical rotation on the motor
shaft, we apply PWM2 100%. I suspect the motor is going to generate
more than the rated 27A?

Yes.

If my assumption is correct then why is that?

Because the rated current is not the maximum current the motor can actually generate. Maximum current is limited by armature and brush resistance. Since this is very low, the current can be very high. In motor mode this corresponds to applying full voltage while the motor is stalled. In generator mode it occurs when the motor is running at full speed and a short is applied across the terminals. In both cases current = voltage / resistance, and all the power drawn or generated is dissipated inside the motor. Unless the motor is grossly oversized for the job, this will quickly overheat it.
A PMDC motor's rated current is generally somewhere between 2 to 5 times less than the stall current. Why? Maximum possible power output is achieved at approximately half the stall current, when half the input power is dissipated inside the motor and the other half is delivered to the load (ie. motor efficiency is 50%). There is no point rating a motor higher than this because the power output at higher current is lower. To achieve higher efficiency the operating current must be less than half the stall current, and then the motor doesn't have to dissipate as much power so it can be made smaller and lighter (important for a traction motor).
What this means in practice is that you can draw several times the rated current for braking by applying a low resistance shunt, so long as the vehicle slows down quickly to limit motor heating. If the vehicle can accelerate from a standing start at full throttle (with no current limiting) without overheating the motor, then it should be safe to brake it with a low resistance because the current curve should be similar. If the throttle has to be applied slowly or current limited to protect the motor, then you should tailor the braking to draw similar maximum current.
voltage, current, torque and speed in DC motors