It does keep coming back to the central conundrum: at what point would she have realised something was 'off'?
Yes, she may have got into the car willingly while he checked out her story. I think many of us may have done that, if instructed.
If he had then said, 'ok I'm taking you to a police station for further questioning' I suspect alarm bells would have rung. That would have seemed excessive for a fairly minor lockdown breach.
If had said, 'I'll drop you off at your flat', that may be more understandable. But what isn't understandable is why nothing happened (as far as we know) at the point at which he clearly wasn't taking her to the flat. At that point, she could have fought back. Of course, for all we know, she did - and was subdued at that moment.
He either had to subdue her as soon as she was in the car, or very quickly into the journey - because even if she got in willingly, and even if she was comfortable at first, she must have realised something odd was happening fairly quickly.
The tazer situation does seem unlikely, and we'd possibly have heard about this if such an event had been uncovered. But is it remotely possible that he had access to an *unauthorised* tazer, and that he used that? If her body was so badly damaged that evidence of tazering couldn't be ascertained, and if he hasn't admitted to it, and if it wasn't caught on camera, and if he successfully disposed of the evidence - then presumably they couldn't charge him for it.
But that doesn't mean it didn't happen. And in some ways, it seems less unlikely than "she got into the car willingly and remained compliant all the way to Kent." JMO.