ok, ok if you insist, even though you did not answer my question correctly.
From where does the GB get the authority to tell JWs which blood parts they can accept and which they cannot?
They have the authority given to them by Jesus. Jesus is the head of the Christian Congregation.
How do they know so surely, e.g., that intact red cells are off-limits but hemoglobin extracts - essentially red cells without the cell walls - are ok.
It really is just common sense. The Bible says "abstain from blood". It doesn't say in that scripture "do not eat" or "no transfussion". Of course not, it was a general statement. Abstain is understood to also include transussions. That is simple, no blood transfussions. Then scientists started seperating components of blood, well now the GB had to address that issue. So basically, Witnesses will not accept the 4 major blood components. After that, well the GB stays out of it.
So, can any JW explain how the GB is able to decide these life and death matters if (a) such things cannot be found in scripture and (b) the GB is only inspired in the sense that the Bible was inspired and so they base their beliefs only on the Bible?
The GB decides this matters based on scriptures and the authority layed on them by those scriptures.