The Nature of Scepticism

No.
I don't think you have thought this through sufficiently. We are developing Theories through empirical observation about behaviors that we "assume" (axiom) for every Effect there must be a Cause.
Hence we point to each Cause as supporting our initial axiom.

?
Galileo used the axiom that "Seeing is believing," (i.e.; empiricism).Socrates, of course, doubted that postulate and said, "Belive nothing you hear and only half of what you see."

Isn't that EXACTLY why Feynman says the Wave/particle observation is the fundamental issue for Physics in this next century???"Because it demonstrates the fundamental limitation of the ability of the observer to predict experimental results, (Effects), Richard Feynman called it "a phenomenon which is impossible... to explain in any classical way, and which has in it the heart of quantum mechanics.""In reality, it contains the only mystery," (to our premise of Cause/Effect????)The very heart of modern physics is this threat to the initial science axiom, of Cause/Effect.

You need review the meaning of Empiricism, first.And then, yes, as you expand upon the simple meaning of relying upon observation, not naive at all,... we can/could add many of the sophisticated reassurances that confirm that we are, indeed, seeing what we thought we saw, like asking peers to review the experiement.Nevertheless, Empiricism is exactly what I said it is:empiricism /emˈpirəˌsizəm/
Noun1.The theory that all knowledge is derived from sense-experience.
2.Practice based on experiment and observation

Now after having brought you up to this point, in regard to what constitutes "grounds,"...... I answer your OP again.Russell ignored the necessity incumbent upon himself, to agree to some axiomatic discipline wherein his scepticism could be put to the test of a Proof.Hence,
1) No, it is not.2) No, it is not.3) Yes, it is paradoxical that Russell would suggest he is sceptical while ignoring the reason we have developed disciplines exactly for the purpose of showing people like him how reasoning can establish truth.
An yes, it is subversive to humanity to support the contention that everything can be doubted at a personal and subjective level simply because one refuses to ground his thinking in one or another of the disciplines of Knowledge.
That is to subvert the very premise that we can think our way through life.

Why would/should one care what his adversary in this discussion thinks about the arguments that corrected you in every case?I consider the matter closed from this side, and you may wallow in your attempts to resurrect any face serving come backs, imho.