JMO
But I think the home security camera had picked SE walking past Poynders or even WC car going past and then going back again. The London Bus CCTV camera had captured WC and the car. Something has happened in that time frame.
WC is obviously a built/ larger (animal) so my guess is that he managed to put his hand over her mouth or even possibly injured her whilst she struggled, deeming her unconscious- hence he could move her into his car. Maybe he could have tied her up, where she couldn't really move and the child lock was on in the car, so even more of a chance of not getting away. I don't like to think of what would've happened when getting towards the countryside.
This may be his first murder possibly...He's got rid of SE and then come back home to see how he can get rid of the evidence. He may have lied to the wife- EC, by saying something along the lines of hit and run etc and she's scared that he'll be taken away/ arrested, so has said she'll help, or that he's forced her to. EC works in a lab- chemicals (lye) available which she's got for him, if she has been going to work during lockdown.
He may have chemicals at home (lye) as he's a former mechanic and maybe this helps to get rid of some grease for work that he does in his garage etc, so he's used this...
If the story is true of an ambulance coming over to their house- maybe it was because he's burnt himself with the lye or whatever. And this has raised further suspicions with the ambulance service. Maybe he got hurt in a struggle and has waited a while before calling the ambulance.
At the end of all this, I really really feel sorry for the poor children (maybe even EC, depending on how much she's involved and how he may have been during the relationship). I don't understand how anyone can do this, let alone a father himself of a young daughter. Surely, it would cross your mind that what if this were to happen to her, he wouldn't like to be in this situation as he family are. But he's a monster so. I pray his kids lead a normal life and will not be affected by this.
This is just my opinion to what may have happened. I don't think he will ever confess.
I hope SE can be found, I hope by some miracle she would be alive, but seems less likely. The body found may be hers. Whosever it is RIP.