Nobody knows who was asleep/awake. the twins may have woken up but went back to sleep when things quietened down.
Sheila had to be as quiet as possible while shooting the first twin to prevent the other from waking.
The silencer was attached when Sheila was bludgeoning Neville's, possibly already dead body, in the kitchen. She may have imagined his voice taunting her.
As for the blood:
a, It was a mixture of June and Nevilles blood [maybe some of the twins blood too].
b, Sheila abused herself with the silencer.
c, The 'blob of jam' blood of AE's testimony fell or was placed inside.
d, Sheila managed to shoot herself with the silencer on, possibly in or near the gun cupboard, and the bleeding wasn't initially so bad, enabling her to put the silencer away and walk upstairs. Venezis states the blood flow could be halted with the fingers held to the wound.