I think there's a very clear reason why none of the FH's acted up for tens of thousands of years, before Baam came along.
They might not LIKE that he's the king, but they aren't acting against it.
And I'm not sure you have any basis they could have ever done anything about it.
Hence, they are subordinates. Of sorts.
If only in title, they aren't the king while Zahard is. That fact alone imparts a subordination to everyone not the king, which includes the FHs.
Rei could be his friend, but, again he also refers to his friend as the king.
Maybe because the title off king off the tower is a futile title for a irregular ?
Sure they are not against it , but maybe they just don't care ? You know not all off those irregular have ambition , zahard seemed to want to be the king , but let us be honest , the king title for now only bring prestige and a FH can have most off what zahard can have .
It could just be , that they don't care because the title don't matter in the first but zahard is the strongest and want that title so why not let him be ? For example Rak is the " leader " or was accepted as that mainly because he wanted it .
Or maybe they made a bet to figth and the last one standing is the king .
Honestly for me the king is a rubish position for a FH , so even if they let him be the king it doesn't really matter .
I mean it isn't me who have to prove that FH can't do anything against him , but you .
This chapter show that Zahard consider them seriously and as serious problem that he has to be careful with .
That is good to you , but my point was that zahard and FH relationship wasn't that off a mere king and his subordinate .
If you point is that the tower gave him the king title i agree .
But that isn't my point , my point is that the dynamic between the FH and zahard isn't that off a king and his subordinate and that it is much more complicated than that , with gustang considering zahard and other FH as friend for example .