Most of the 60 + side effects of Haloperidol relate to the person being drowsy & not having the physical or mental capacity to perform basic tasks. There is no reason why Sheila would be an exception.
Not surprisingly, the evidence from Sheila's best friend a week before the massacre, saying Sheila needed help getting off sofa's & AE saying Sheila could not put sugar in coffee show Sheila was suffering the side effects.
On the night, the WS's of Bamber & PB also show Sheila was 'unresponsive', 'not paying attention', drowsy, weak, 'not talking', tired & 'going to bed'. Which again shows she was suffering the side effects.
Obviously there is no possibility Sheila could have committed the massacre. However Bamber would be aware that Sheila would be easy to control later on.