You essentially have it all backwards.
You can not control power with laws. Do this thought experiment with me: Imagine you had a control panel where you could just make anyone do whatever you want. That is what power is, the ability to make other people do what you want. Now, how is a law going to help against your power? It can't. Even if you allowed someone to enact a law that forbade you from killing people (obviously, you could use your total power to prevent that in the first place): If you can in fact stop anyone from trying to enforce this law on you (and in this thought experiment, you obviously can), then the law is completely inconsequential.
Now, it still seems like there are laws that control power, doesn't it? Like, say, the US constitution. But imagine the same thought experiment, with the US consitution in place. The constitution would not in any way prevent you from doing whatever you want, would it?
So, how come that it still seems in the real world that the US constitution controls power? It's because noone has the de-facto power to ignore it, and there are many institutions and the will of the governed that work to keep it that way. That is how the constitution actually works: It works by a consensus of those who have power wanting to keep the power away from their opponents.
You are mixing up nominal power and de-facto power. Laws can describe nominal power structures. But if those who wield de-facto power don't agree, those laws are completely inconsequential. A law is only as good as the will of those with the de-facto power to enforce it.
This is why you can not control a tool that gives de-facto total power with laws: Those who wield it have no reason to care about laws. A law prescribing separation of powers is worthless if one entity has the de-facto power to ignore it. So, the separation of powers is not actually a result of the nominal power prescribed by law so much as it is a result of a system of governance with a huge number of participants each wielding a little bit of power and using that power to keep power away from others--where all those participants may well be inspired by the law to keep it that way, but the point is: If they stopped enforcing this balance of widely dispersed power on each other and allowed too much power to concentrate in one place, the way back can be hard.
Essentially, democracy is one huge exercise in keeping de-facto power from concentrating. Seriously, read the constitution of any free democracy. Read the universal declaration of human rights. The whole point is to put in hard work to counteract the natural tendency of power to concentrate, because of the bad experiences humanity has had with that.