But how can the authority of the Sages be grounded in “make a safeguard for My charge,” when that itself is their interpretation of the verse “and you shall keep My charge”? Seemingly, you can ground the authority of factor x in factor z, but not have that same factor be the source of its own authority.

Easily. Just as the legislator determines his own powers. In our case it’s even easier, because they only interpret, they don’t determine.

Yes, but I can interpret it differently, and therefore understand the verse differently from how they interpreted it (“and you shall keep My charge”), and then I won’t be bound by their obligation.

Correct. You can also refuse to accept your obligation to the Torah or to the laws of the state.

I’m asking seriously, so why should I accept their authority if it isn’t actually compelled?

And I’m answering seriously: because there is no legal/normative system without a body authorized to interpret, update, and legislate.

Noam, Michael has a certain bug in how he understands the concept of authority. He doesn’t feel the arbitrariness of the recursion in defining authority. He has repeated this again and again and does not grasp the difference between “it needs to be such-and-such” and “it must be such-and-such.” The ideal/the desire turns into necessity. Someone wrote to him here that because he’s hungry, it must be that there is a sandwich in the next room; same with your question: the source of authority exists because there needs to be authority. Many have tried, but have not succeeded, in cracking this bug in Michael’s thinking.

How nice that Michael is still alive and can be asked. Fortunate are we.