Well,all I can say is,unless Sheila flushed all her medication down the toilet and disposed of the containers before her death then someone must have found them either at WHF or in her flat!
So, despite that her autopsy revealed the contrary, you're insisting that she bought medication with her to self medicate? If we leave out the law according to Snow, and look at it logically, why would she, even if we accept your belief, bring 'party' meds with her, decline to take them, and flush them down the loo if she intended murder/suicide? I don't imagine she'd have thought it an imperative that she'd be found to be 'clean' at autopsy.
I can't rule out that she MAY have had some back in her flat -perhaps old stock which JB might have made use of?- but having complained that the high dose of Haloperidol left her unable to function, I find it doubtful that she'd take anything which exacerbated that condition.................and actually, as the autopsy revealed only "trace elements" of recreational drugs, it seems she didn't.