Magic power is not based on physical genetic.
In fact the mother's level of magic have the most influence on how much magic power a child can have.
And whether a couple can conceive a child usually depends on how harmonious is their magic power (so yes, Lutz xMyne is dead, he doesn't have any real magic power, she has enough to overpower a royalty)
However, people with consuming are slightly different from nobles in a few things.
1. Their aptitude is weak but consist of all element, whereas nobles' aptitudes are usually a mix of their parents' (the closest magic power has to genetic)
2. A person with consuming is easily influenced by magic power of others, unlike nobles, however the effects of the dyeing does not last long.
2.5 Unless they're those with "Life God's Marks" i.e. reached a state of 'death' but manage to survive. These people retain the effect of magic power that dyed them, making them seem just like nromal nobles after you've cured the magic stone fragments inside them.
I don't know about treatment of the consuming in the past (it seem pretty well given that there's data on them like that) but currently at least, this country is in a decline.
The cause can be traced to losing their history over a loooooooooong period of time.
This whole country is actually made by the Gods, as a haven for anyone with magic power.
Back in the past, the Ruler would also be the Temple Head of the Central Temple, and a new Ruler is selected from the strongest candidate who've passed the trial by the Gods.
Then at some point, a queen wanted her son, who totally isn't qualified, to be the next Ruler.
So what she did was make a copy of the special magic tool that's used in controlling the functions of the country itself (drawing domain borders, open/close border gates, etc) which can be used by just about anyone, thus the son was able to succeed her.
Then that son just pass on that magic tool.
Over time people forgot there was the whole trial of the Gods, the Temple became dumping ground of 'excess nobles' and is no longer the place of worship it used to be.
And then recently there was a coup that resulted in said magic tool being lost (and a lot of dead nobles, so suddenly there's a massive drop on available mana)
Without that magic tool, the current royal family have no way of running the magic controlling the country, and all they can do is (inefficiently) try to sustain the magic covering the country.
Myne ends up reviving the old culture through a lot of her own actions and unintended consequence of her own actions (for the most part, it's related to her wanting to read)
As for creating magic...kinda unlikely.
Without assistance of the Gods, a person can at best discharge their magic power out in a wave that hurt those around them.
However compare to the magic with the Gods' blessing/Staff, it's like comparing a child's sand castle to the sky scrapers.
The difference is huge, and basically impossible to compete.
To give some example of how different it can be.
At one point, a black monster (similar to toronbe, but a beast type) shows up at school.
After they were able to defeat it, Myne went back and heal the devastated land in mere hour.
Later on a group of priests from Central pointed out how impossible this is, since it'd take several of them taking turns for DAYS to complete.
Myne's explanation is because everything from school (Staff, lessons, blessings, etc) makes her magic far more efficient than what the priests can do.
Another time, enemies snuck into the school and got their own Staff (there's a lot to explain if I go into details so I won't)
The fighting between them and the knights were fierce
Until Myne's group figured out how'd they register themselves to be able to get the Staff
then they go and annul their registration, making them unable to use the Staff anymore
and immediately the enemies were subdued
Now you might point out that queen made a copy of the magic tool
Yes she did, but to do so she required the Staff, i.e. you need the Gods' help to begin with