A LOT depends on when the silencer was found (and this is a subject in itself, with two ends of a spectrum ranging from 3 days to a month!).....
But:
1) If it was 3 days, is that enough time for Ann and David to 'plot' Jeremy's downfall? and, how did they get around the problem of having a witness see them find the silencer? Did they plant it earlier, only to 'discover' it in front of the witness (accountant)?, or did they find it, then have time to add more evidence when they took it home?
2) Could they have 'colluded' with Mugford? It seems unlikely because Mugford was still 'with' Jeremy at the time, and appearing as the loving g/f. Could they have ever banked on Mugford giving her story without collusion?
However, IF the silencer was found not three days later, but a month later, then this could paint a different picture.
3) Could they have obtained the DNA / blood samples needed AND with enough certainty of that forensics would determine?
I suppose they could have gathered blood from the stains in the rooms, but not easily (dried), and tricky to be sure of whos blood was whos, even knowing final body positions.
4) Was it a risk worth taking? 3 days to take this huge risk. What it forensics discovered it was framed evidence? they would lose everything - quite a gamble I'd say.
5) Did they know enough about forensics to fake the evidence convincingly?
6) It seems (to me) horrific enough that 5 were dead, and even if there seemed to be disbelief at the whole affair (pure shock) and the 100% responsibility of Sheila, it's a heck of a jump to thinking Jeremy did it. It's think most folks would say "no, it couldn't have been, he's an idiot, but THAT bad??" etc etc.
And yet, Ann's relentless 'efforts' could be seen as distinctly quick to judge Jeremy. That impression could be wrong, and it could actually have been 'a small doubt' in the early days, to finding the silencer, and then watching Jeremy more and more, leading to a much firmer distrust of him.... but still, what was it that led Ann in particular, to almost instantly doubt the suicide story?
Again, I repeat, some of the above very much depends on when the silencer really was found, but for the moment, I'm going with 3 days later.