I have quietly been reading all the posts and a few things stood out.
1. Pre-meditated/planned abduction/murder: I didn't think so to start with, but I do now. Stirling had already threatened her and I think he planned the pseudo/sudden holiday to make it look like he was elsewhere. Oddly, there is a Brit Netflix show The Fall where part of the plot is similar - he breaks into houses of women living alone - first watched their regular patterns, then lay in wait inside many times, or broke in when they were unsuspecting. He keeps an old girlfriend hostage in a disused country house. He abducts her, bundles her into his car, at one point has her in the boot of his car. Keeps her hostage in the country house for weeks and tapes her mouth. Keep wondering if Stirling got ideas from it - copycat style.
2. Suffocation as cause of death: I think this is very possible. Someone suggested it. I do think he taped her face/eyes so she couldn't see where she was being taken. And taped her mouth and inadvertently suffocated her. If this is what he told police, they will want absolute evidence as one is murderous intent and the other is not. Strangulation would be obvious. Suffocation would mean that she was alive/kidnapped/bundled into the van injured and possibly died through suffocation which would account for the 6 months needed for specialist forensic pathology.
3. Significance of scream: I think he broke into her house while she was sleeping in preparation of her next shift. Someone suggested this too. I agree. She already knew him to be a threat so it's too hard to believe she would knowingly let him in. She'd expressed concern to colleagues that she could go missing. She wouldn't have let that kind of threat into her home. She would have called the police, not let him in if he'd simply knocked on the door and asked to have a word. In my opinion, he has let himself into the house while she was asleep, or lay in wait for her.
4. Police evidence - telephone: Police have said they have this along with CCTV and plate identification. Likely to be text exchange with his dad, i.e."Police keep on at me. Can you just say I was with you." However. it is very possible that at some stage, Stirling had also sent Samantha a threatening or abusive text message which resulted in her letting colleagues and potentially even police know. Remember, that these things are hard for police to do something about until something happens. Many a stalker gets away with insidious behaviour for years before they cross the line that police can take action on.
5. Motive - some sort of unrequited love/perceived rejection: I still believe that Stirling probably became obsessed with Samantha and was somehow rejected by her. Clearly there is something crazy about him for him to have done this - right down to taping her eyes/face. It all screams man gone crazy.