1. In my head, I cannot contrive a situation where a fuzzed packet causes a clearly unusual power consumption measurement, but the controller still persists to appear to work normally. Hence, I suspect a functional test that sends a bunch malformed packets, and then runs a standard test routine, would be much easier and much more likely to be successful.
2. If you want to persist with monitoring consumption current of the microcontroller, I repeat the recommendation already made: You need to measure the current going into the microcontroller, not the entire PLC. Measure the current consumed by the microcontroller (break the VDD trace), and then select a resistor value that produces are certain, acceptable V=IR drop when placed inline. Then use a uCurrent to boost the reading nicely, and feed that into an oscilloscope.
Remember to not blow up your oscilloscope, see the EEVBlog video "how not to blow up your oscilloscope". In short, connecting the ground clip to one side of the resistor will cause all sorts of problems. So the PLC will need to be floating, or you'll need differential probes, to perform the measurement.