The fundamental problem with that argument is that it's assuming you can appeal to your own belief about what is moral and use that to override the claims of the martians even though they have vastly superior brainpower.
Okay, this is going to be a long response over such a short sentence, so... sorry about that.
I think that we can still draw reasonable conclusions: we are finite beings that are forced to draw conclusions from incomplete epistemic circles all the time. When we do science, we're doing the best with what we have: just so with nearly everything else. Precisely because of these limitations, we need to be aware of epistemic traps that are impossible to evidence ourselves out of once we fall into them. Let me give another example.
Consider a case similar to David Berkowitz (a serial killer named Son of Sam who, the story went, took his orders to murder people from what he believed to be a spirit possessing his dog). Let's say we have a serial killer that takes orders from a voice claiming to be God, just to get a closer analogy to the point here. Let's call our dude Jack for brevity.
Now, Jack affirms propositions such as
(1) God exists
(2) God is omnipotent
(3) God is omniscient
(4) God is omnibenevolent
So far so good for most people. Jack also affirms a consequence of these propositions that
(5) It's possible for God to communicate with me privately if God wanted to
I think most people would still grant this without controversy. However, what happens if Jack starts hearing a voice that tells him to kill? He's now adopted a proposition that many people
would find controversial (okay a set of them):
(6) God is actually speaking to me
(7) God wants me to kill Susan next door
Now, here's part of my point: why is (6) controversial, let alone (7)? After all, as you point out, we don't know everything: who are we to say God can't do as He pleases, and perhaps speak to Jack. Is it reasonable for us to say that (6) is probably false? I think it
is reasonable, and I suspect that you do too (we will see what you say). But how can this be so when we are finite beings with finite epistemic bubbles and there is so much we don't know, and God (given the other premises) really
could be doing (6)?
I could launch into a series of explanations for why it's
reasonable to doubt (6) is true, but I'm sure you can imagine them so I will not have to be so explicit unless you make me: it isn't parsimonious for instance to bring God into the picture when when an alternative explanation is
(6') Jack is experiencing a hallucination
(Supported by premises that humans sometimes hallucinate, etc.)
Now, if you agree that we can be reasonable in doubting the truth of (6), then I think you are on track to seeing why we can be reasonable in doubting that apparent malevolence by a more intelligent being is really, somehow, secretly benevolent. We can be reasonable in the absence of
all information because we're forced to, it's routine for us to have to do this. N'est-ce pas? And it's reasonable for many of the same reasons: believing there is a secret reason that an apparent thing, even a blatantly apparent thing, really isn't that thing (say, that apparent malevolence is somehow inexplicably benevolence) isn't very parsimonious.
So, I haven't been arguing that we can know in an absolute sense that apparent malevolence isn't secretly benevolence when performed by a more intelligent being: I have been arguing that it's
reasonable to doubt that it is.
Moving on.
Let's talk about (7), where Jack believes God has told him to kill Susan next door. At first, Jack finds this to be incongruent with (4), that God is benevolent. But Jack has an idea!
(8) God, being smarter than Jack, has an unknowable reason for commanding Susan's death that is in congruence with (4)
But isn't this a problem? Can't Jack make any infinite number of premises in the same style of (8) that justify literally anything? Once Jack shunts away responsibility for reasonably assessing the situation ("God might have a reason I can't know"), Jack could literally form any premise of this form. Nothing could ever convince Jack, once he's accepted a premise with the same form as (8), that premise (4) might be false:
nothing could
ever convince him.
This is the epistemic trap. So, knowing that (8)-style premises are literally inescapable, is it
reasonable to form an (8)-style premise in the first place if we know we can never get out of it once we adopt it? I don't think that it is: I think this is some form of meta-epistemic argument about the styles of premises we should adopt or more aptly
avoid adopting because of their inescapability.
In fact, we can build any kind of trap like this that we like. Given these premises:
(9) An unie exists
(10) Unies are omnipotent
(11) Unies are omniscient
(12) It is the nature of unies to abhor blue M&M's and to seek their destruction and removal from planet Earth
We might look around us at planet Earth and see a lot of evidence that is incongruent with (12). We might reasonably doubt that the collection of premises (9)-(12) possibly describes reality given the contradiction. But wait! We can make an (8)-style premise and fall into an epistemic trap if we like:
(13) Unies, being omnipotent and omniscient, could have an unknowable reason for allowing the blue M&M's we see on Earth to exist for right now that's in congruence with (12)
We're saved, right? As long as we make something smarter than us in the premises, we can literally make any kind of (8)-style premise to say anything we want, anything we can imagine, and we can never escape from it no matter what the evidence is after we've adopted it!
Is this epistemologically sane to do? Is it reasonable? I don't think that it is. I think that we have to make this kind of meta-epistemic analysis to understand that there are some kinds of premises that are traps that we should not adopt. Furthermore, I think that we have to realize that we can and do make reasonable beliefs about things for which we are not omniscient: such as doubting Jack is actually hearing the voice of God in the face of the evidence that Jack may just be hallucinating -- or doubting that all the suffering God would be culpable for (like physical suffering) is really somehow benevolent when it's normally malevolent to cause suffering when there are alternatives.
I will respond to the rest soon, or another time, depending on when I'm going to bed here.
Edit: I wanted to put it like this. When confronted with something smarter than us in the premises, it feels like we have two options when we find an apparent contradiction with the premises and the world that we see:
a) We form a reasonable belief even though we're not omniscient based on what we do know, like when we reasonably doubt Jack is hearing the voice of God telling him to murder. (I would also argue: like if we doubt all the physical suffering in the world really has some secret benevolent explanation).
b) We form a premise (8)-style trap and just let ourselves fall into it, even though we know that we can form any kind of (8)-style trap we want arbitrarily, even though we know we will never be able to escape it no matter what evidence presents itself.
I do not think (b) is wise. Do you?