Yes, and I've said why. I think it would take a very devious mind to think up such a plan.
The chance of succeeding wouldn't have been that low. They probably wouldn't have known about all the enzymes in blood and all that stuff.
In my view they could have attempted point 11 themselves. However points 1 - 10 were essential to know & only the police could tell them.
1) Contact shots during the massacre.
2) Blood already on rifle nozzle.
3) Photos of aga.
4) Lenght of rifle with silencer.
5) Lenght of Sheila's arms.
6) Other evidence showing guilt/innocence.
7) Police checks already carried out on silencers.
8 Where to get Sheila's blood.
9) How to insert Sheila's blood.
10) If unable to get Sheila's blood, what was her blood type.
11) How to effectivly scrape the aga to replicate a scrape during a random fight.