I'm not sure if the Ackerman & Asian clans fit into this maybe they are remnants of the 7 titan clans conquered by the Marley or maybe something else.
I wonder how the Ackermens super soldier abilities fall into this. Perhaps it is carry over from one of the 8 titan clans special abilities that was able to be passed on by genetics.
Didn't Kenny tell his grandfather something along the lines of "We used to be the king's most trusted people, and now we've been reduced to shit and are persecuted"? Perhaps the first king (Fritz) knew that the Reiss family was only supporting the walled kingdom because they wanted to take over the throne (After all, there seems to have been a civil war between the eldian clans). Knowing that the Ackerman and Asians were inmune to mind wiping, and also that ackermans were very skilled fighters, he chose to keep them close to himself all the time (think of the ackerman as the king's personal bodyguards and advisors).
But somehow the Reiss still managed to take over, and the persecution of the minorities started (so that no one would know the truth about the outside world AND about the "fake" royal family)