He has come up with a theory for an alternative and plausible on the face of it--unless the police do have other pieces of evidence that they have not released and that is a possibility we should not ignore--suggestion as to where SJL might have been heading that day.
That's all he has, it is a theory based in some facts--that SJL did have reason to go there, and she could not have gone there at 6pm. From there he makes a leap that SJL DID go there, she was killed there (or had an accident that was covered up as if it was a murder. I really find that hard to believe). Is that worth looking in to? Again on the face of it, it seems to be because those are facts, and all other avenues have not really worked out. THe Mr Kipper angle appears to be a bit of a dead end. No one could identify him and no one saw him on the street where the car was abandoned. There is nothing to link this figure to someone in SJL's life. So yes it seems it is worth exploring even just to rule it out if it hasn't been ruled out already. Assuming something didn't happen without checking it is not a good way to investigate.
The other bits that DV puts around the above theory are conjecture, and can't really be proven from the bits and pieces we have to work with. The keys story sounds plausible but also, there is no evidence really she didn't take them with her. And if she was making up a fake viewing to get out of the office, why not take the keys to make it look real? What would she have said when she got back, sans keys, and with no real ability to enter a fake client into the system they were supposed to use? You'd just take the flipping keys!
The other presentation of CV and his ex "acting suspiciously" I think are him editorializng and presenting this material to achieve a certain effect. AL being upset could be for a million reasons maybe he is convinced JC did it and isn't open to any other theories.