Personally I always believe in Occam's Razor until a more unlikely scenario can be proven.
The trouble with this case is that it defeats Occam's Razor. The simplest scenario is that she went to 37SR and was abducted.
However, we don't know how she got to 37SR. If she drove her car there then she or someone else drove it away as well as his own. On the basis of other statements, her car went in completely the opposite direction from the office and never moved again. If she was driven to 37SR from 123SR by someone, then the sightings of her waiting outside make no sense.
We can be pretty sure SJL never went inside 37SR because a/ that would be stupid by her abductor and b/ there'd be traces of her. He could keep his hands in his pockets, but if he does that, she has to leave not a single print on any door, door handle, cupboard, light switch or banister rail that she touches.
If she never went into 37SR, then she never came out. This means that HR's account is wrong because he said he heard them do so.
If we grant that HR saw somebody at 1pm and it was SJL and Mr Nobody, then we have that one sketch to go on, based on a few seconds' glance. We don't really know who else saw what. Nobody else is sure of what time they saw whatever they saw, so the various "corroborating" sightings could be of MG and SF.
A simple-ish scenario that fits some of the facts is one where she goes to 123SR, parks sloppily, meets someone by arrangement, goes back to 37SR, then goes elsewhere with said someone who kills or restrains her then goes back to her car. This allows witnesses to have seen her car in 123SR and her outside 37SR.
Unfortunately it doesn't explain why she met someone at 123SR and didn't go in, met someone outside then was driven to 37SR and didn't go in there either.
It would allow the various sightings later that afternoon at / near 123SR to be correct, but it requires that the BW sightings and the Fiesta being driven erratically near Shorrolds all be discarded.
All this adds up to quite a lot less than any easily-available simplest explanation, and that's before you even consider who an abductor might be. The simplest solution - that it was JC - fails the sense check of how many others it might have been. The simple solution is not that JC was this man because it's what he'd do. The simple solution is that it was done by whoever out of everyone possible was most likely to do it.