A "Demon" even God cannot excorcise.
God cannot do anything that is logically impossible; like draw a square circle, or make something red all over and green all over, or create a divisible metaphysical unit. And it could be that something cannot be a finite cognizer and have the resources to overcome sceptical cases. For example, it may not be logically possible for a finite mind to be able to assure itself that it is not a brain in a vat or under the spell of Descartes' Demon. Many traditional arguments against theism from rivil metaphysical systems can be understood as versions of sceptical arguments with respect to local topics. For example, that we must reject moral truths or color impressions because they have no place in a world of physical laws, can be seen as a scepticism about moral truths and colors based on having an alternative conception of the world which we cannot be assured is false. But perhaps our inability to assure ourselves under these circumstances is an essential feature of our finiteness as thinkers, so it is quite possible that we can do nothing about it. But since this is an essential feature of the kind of things we are, there is also nothing God can do about it. Intuitively, it seems to me that the only being that could overcome such a lack of assurance is God Himself, andthis would explain why He could not create other such beings.
Let's move our attention from Russell in the dock of heaven to Clifford's ship owber. According to Pro. Clifford, suppose a person owns an ocean liner and wonders if it is still shipshape. However, this shipowner lets the ship go full of passengers without inspecting it and the ship sinks from neglect of maintenence, killing all on board. Clifford's compelling intuition is that the owner is responsible for the deaths of all those people in virtue of failing to do a sufficient investigation of the seaworthiness of the liner, and further, that he would still be responsible even if the ship successfully made the voyage without sinking. From this, Clifford concludes that it is always morally wrong to believe something without sufficient evidence. But does sufficient evidence include being able to answer sceptical questions? It would seem not since imagining the ship owner doing a maintenence check and passing the ship because of it, and then asking "But how do I know the ship is really there?", would undermine the grip of Clifford's case for the sufficient evidence principle; the ship is definitely seaworthy whether or not we are justified in believing in its existence. So sufficient evidence for sending out the ship falls short of dispelling sceptical doubts.
Russell thinks that God ought to be able to provide sufficient evidence that God exists as opposed to agnosticism being true. But ought implies can and we have a reason to think it might be that God cannot meet Russell's request. This is an additional consideration in behalf of (4) in the previous post. Further, we cannot assure ourselves that God has not been doing all He could do to make himself known to beings such as us, as characterized by traditional arguments and evidences. In fact, once we begin to think that the request for defeaters of sceptical doubts is illegitimate because impossible to satisfy and also unnecessary for religious purposes (as in the ship owner's case), we may think that what is availble is not only legitimate but even super-abundant.