Seems your beef is with SN, SN as the late delivering carrier is responsible for the recovery. And EU261 compensation

Suspect you are think about downgrade compensation on UA metal due to oversold situation, would not apply here.

Your PlusPoints should have been refunded

As it was a 016 ticket, UA could have taken over the recovery but that would have been unusual as the responsibility / expense was SN responsibility.

It is hard to understand how there were no SN (Brussels Airlines) agents available at BRU