I can't remember the trial - I thought it was Westerfield - where a Court TV psychiatrist talked about how a good percentage of sex crimes against children are not committed by pedophiles (people whose main sexual arousal comes from children) but people who are "polymorphously perverse." (Freud's phrse, IIRC).
Anyway, he said that such people are often aroused primarily by adult heterosexual fantasies, but when they can't accomplish that type of sex, they transfer their arousal to the nearest object (porn, child, animal, etc.)
The theory would be that Westerfield was actually aroused by - and perhaps seeking contact with - Mrs. Van Dam. Danielle was a second choice.
I'm rather surprised that Dusek didn't try to do something with expert testimony along these lines. DW gets jilted by his GF. Then BvD and Danielle come around with the cookies and get invited into the house. Shortly thereafter, he runs in BvD flirting around with other guys at a club; she shows no interest in him. He leaves, feeling frustrated, and knowing that Mrs. vD isn't home looking after the kids, etc.
That still wouldn't explain how he could have got to Danielle without leaving any evidence behind, but it could explain why he might think about going after her.
Not my theory -- just keeping the thread going. ;)