On the question of what the defence's line is, I note that when the screams were first mentioned by the prosecution as evidence pinning down the time of the assault (before the jury had visited the crime scene) the defence seemed to cast doubt on the prosecution's timeline and therefore on whether the screams heard were in fact Joanna's. But later on, on the issue of how long Joanna was strangled for before succumbing, they quoted the relatively short duration of the scream-scream-thud sequence (less than 10 seconds) and badgered the pathologist into stating that he couldn't be certain that this did not represent the strangulation period.
Clearly they can't have it both ways. The screams can't have been Joanna's when they are convenient (suggesting a short strangulation period) but someone else's when they are inconvenient (suggesting a very early attack with little time for any provocation of any sort).
I may be wrong but this looks to me like a defence that is not out to sell the jury a clear alternative version of events but wishes simply to cast doubt (even incompatible doubts) on individual aspects of the prosecution's case. And if I am right that looks very weak to me, though it may be the best they can think of. After all, VT knows when he killed her. He knows whether she was in the boot when he went shopping. By implication he has a very good idea whether it was Joanna heard screaming or not. I hope the jury will be expecting to be told a credible version of events that contradicts the prosecution's case and that any vague calling into question of details without giving a consistent opposing account will be dismissed.