Spoiler…..
Well perez forced the king on this issue, before marrying Tia. He knew that Tia loved Lombardi so much that he made the king signed and declare that Tia Lombardi can be the crown princess and the head of Lombardi at the same time. They do have children (4 I think, I dont remember) one will inherit the throne and one that will inherit the Lombardi duchy.
I didn't read the spoilers but after ML becomes the king, I'm sure he would do anything to make it happen. Who knows, maybe he's already on his way to make that happen, I mean, he's already attending that Meeting of Important People. He just need those Angenas to fall down even more so that he can have more say in the law.
As for the duties and Queen and Head, she can very well do both together. As long as she has a lot of assistance, just like how Clerivan is handling Pellet Company. As long as she has the final say, did some paperwork, and show her face in social gatherings, she can absolutely multitask and do both.
if perez becomes the king and tia becomes the head of house lombardi, would it even be possible for them to marry without tia losing her status of head of lombardi? (considering queen is a higher status than a head of household, technically.) or would she be both a queen AND a head and they could technically pass on the throne to one child and lombardi to another? maybe i’m just overthinking this