Can we be certain of consciousness? How can we be certain that what we perceive as reality isn't some demon feeding sensory information to our brain?

That would still be consciousness--or call it 'perception' if you like. But yes, objectively speaking, this is as likely to be the case as any other explanation.

We can be certain of - "something's going on". That's about as far as we can take it, but it's something that isn't nothing.

It can be proven. Look down at your navel. Did you know about that lint? It was outside of your consciousness.

Maybe that lint was it, but probably not.

That doesn't prove it - maybe you just decided there was lint there when you looked down.

I often think of GE Moore when I see this argument: https://en.wikipedia.org/wiki/Here_is_one_hand

That argument simply assumes its conclusion by arbitrarily labelling hands as 'external'.

The 'existence' of our hands is perceived by our senses and filtered through consciousness. There is no way to prove that they actually exist outside of our minds.

Things that cannot be "proven" still can have vastly different probabilities, according to prior observations.

There are no observations that are not products of consciousness, so there are no prior observations in this case.

I think therefore I am?

I think, therefore I avoid vacuous statements like "I am therefore I am"; moreover, I think I am.

It's raining, therefore it is