As explained you'll get points back without question because you were not given the class of service you booked; however, the amount is generally always less then people expect because of reasons.

I have had to ask for these before on behalf of my family and found if you can politely point out areas for improvement you may find they will give you something extra. I personally think you want to avoid the root cause (pilot given seat) and just treat it like any other downgrade where a seat became unavailable (equipment change, faulty seat). Focusing on the communication issue may be helpful if you explain politely the impact (e.g. didn't get access to overhead space in a timely manner because you boarded last). I'd personally avoid getting into any of the reasons for the downgrade. A downgrade is a downgrade.