Is there any evidence at all that Sheila had suicidal thoughts at any point? If so, when? To whom did she confide? She did say certain things that were out of place, such as "All people are bad and should be killed" (according to Barbara Wilson, I think) and her delusions about her sons, and so forth.
It's a fascinating situation in which each of the suspects, Jeremy and Sheila, said strange things that could be twisted to look incriminating. You have the added layer of realisation that Jeremy himself must have been aware of some of what Sheila was saying and doing and possibly formed a murder plan on that basis, believing all of Sheila's odd behaviour would be recalled and she could thereby be framed, whether alive or dead from suicide.
Sheila could have survived one shot from Jeremy and then told the police: "But it was a masked man that did it! He came in the middle of the night and shot us all!"
Who would have believed her? An interesting counter-factual scenario to consider. Would we now have fora and innocence campaigns for Sheila, locked up in Broadmoor or Rampton? Free Sheila! Or would libel laws put paid to such a campaign? What evidence could avail such a campaign? Interestingly, the evidential position might be reversed in those alternate circumstances: the silencer evidence would become the more important factor; Julie Mugford's evidence could be more open to question, assuming Jeremy confided in her, because her account would be testable against Sheila's.