That is possible, depends on what it is about the crystals that powers their abilities. Could we make them, could we recharge them, could we find a mine? Hard to say. Sadly, we just don't know that much about them yet. Maybe we could examine various ones, from as fresh as you can get to ones that are nearly depleted and see if we can figure out what is being 'used up'. Since we can't examine an 'empty' one, since they explode.
Cornwall seems to have a good bit of power over the elves, but we have already seen that he is willing to break other laws if it will help him, and he could be removed if he pisses off the wrong people, and this goes for anyone we could replace them with. The president would also have a lot of power, but he is not a king, the other branches of government could easily override anything he does or remove him from office as well if he does something they don't like. For that to work, they will need the support of the other branches, or it won't work.
Real life example, when we did it in the U.S., not only did it start a war, but the president that did it got assassinated.
Yes, he is in part responsible for how Maria was treated, very true. But, we can't let our emotions burn any (more) bridges than we need. He is currently working with Kali and Autumn's dads to improve the lives of elves, so while he doesn't do near enough, he is still technically a partial ally. The best way, would be to work with him, help him see where he is wrong, and then convince him to retire and support/endorse our replacement.
The MC is not actually registered as as an owner, and is not a native citizen, if he is a citizen at all, now that I think about it. So he could never be president, though if he passes the citizen test, he could at least run for Cornwall's job. Which, in and of itself is a black mark on his record. But, even if that works out, he is just one guy, and can only look for issues in one group, in this example, Cornwall's. Admittedly, his is the best one, and if you only go after one guy, he is the one, but to do this right, it really needs to be a bottom up campaign.
The other problem with it being the MC, is that he is not the focus of the story, and it would take a lot of time, more than just one event, so odds are, that would probably be one of the girls' jobs, since it is their story, not his.