Why are you apologizing? Did you make reality this way? Are you God?! Or are you just condescending me?
If you think that there is some objective morality that is inherent to the universe, the burden of proof is very much on you to show that, not on me to disprove it.
I'm going to disagree with you on this point, because not only the existence of objective morality, but the existence of object truth of any kind depends on its falsity.
As you say, my brain makes certain a priori assumptions about reality. Among these, are the existence of object morality, the utility of the senses, and fundamental correctness of human reason. My belief in these are the default. If I am to doubt any of these things you must provide me with a concrete reason to do so.
All reasoning proceeds from assumptions and unless we make allowance for assumption, we must assent to solipsism (or something equally silly.) For, if we are to assume that all premises are false, unless proved otherwise, then what premises are true? People who hold the stance tend to make all sorts of assumptions that they don't realize are assumptions. Thus, I will maintain that rather than arguing from a 'nothing is true until proven' stance, that it is better to argue from a 'default brain state is true' until disproved stance.' Which, would then place the burden of proof on you.
* I am 100% certain that you cannot give a definition of "morality" that both meshes with what's expected of the term and that implies the existence of some fundamental objective framework for determining what's right and what's wrong. Just think about it for a while.*
As you state, expectations can be wrong. However, for the sake of sanity, it is better to presuppose that they are less wrong than more wrong. The people nearly universally agree that both theft and murder are wrong, that they disagree on whether lethal force is justified in defense of property can be attributed to bias of experience.
Or, alternatively [and not equivalently, but evocatively]: we're all just particles. You can give any brilliant argument you like about the morality of an action, and my particles are just gonna keep on doing whatever the fuck they want.
Except that I'm not a physicalist. If I were,I would already agree with you.