You make the very good and very important point that the relatives wouldn't have had the knowledge about blood forensics which many conspiracy theories need to assume that they did have.
Although Robert Boutflour's blood happens to give the same tests results as Sheila's, they would have had absolutely no knowledge of this. Without knowledge of blood groupings etc and forensics, they couldn't possible have known that if they planted their own blood in or on the sound moderator that it would be a match for any of the victims!
But when you consider 8% of the unrelated white British population shared SC's (and RWB's) blood groups it is unsurprising that RWB's blood groups matched SC's. Statistically at least one of the regular posters here will share those groups. And at least one member of the jury shared those groups. Oh and at least one member of the CT and the hangers on would share those groups.
Also the drawback phenomenon, even today, is very esoteric. The relatives cannot possibly have known about it. Nor could they have known what images existed of the underside of the Aga surround.
Racing cert - DS Jones - blood outside, paint, scratches and hair
Racing cert - FSS - blood test results inside