I very much agree with "All knowledge must be built upon our instinctive beliefs. If these are rejected, nothing is left".

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I agree less with the second concept .

Russel said "It is of course _possible_ that all or any of our beliefs may be mistaken, and therefore all ought to be held with at least some slight element of doubt.

But we cannot have _reason_ to reject a belief except on the ground of some other belief.

Hence, by organizing our instinctive beliefs and their consequences, by considering which among

them is most possible, if necessary, to modify or abandon, we can arrive, on the basis of accepting as our sole data what we instinctively believe, at an orderly systematic organization of our knowledge"

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He is therefore implicitly asserting that "the fact of systematically organising instinctive beliefs guarantees greater 'gnoseological power" is itself an instinctive belief, on the basis of which to accept or reject other instinctive beliefs.
And it could be. Putting things together, coherence, add something to our knowledge, we can feel it.

However, it is not justified why systematic, rational organisation, should be elevated tosome sort of 'the belief of all beliefs', 'the instinct of all instincts' on the basis of which to select others.
I believe it should be treated on a par with any other instinctive belief. Accepted, as it is, and with the limits it has, and used to decode and know reality without the pretence of making it an absolute or putting it in a superordinate hierarchical above other instinctive beliefs.