It is very interesting that Amy was just happening by and saw this. could she have seen the 'abductor' as well? So where does that leave poor Diane Webster- telling us it didn't open and stay up... or Gerry Testing it. WHO CLOSED THE SHUTTERS AND WINDOW? Tsk!
As I have said previously, following listening to hundreds of murder cases described on YouTube, where you have parents involved, they will protect the name of their offspring. If Dianne had any suspicion her daughter or son in law were involved in any way it is likely, from my studies, I'd say she would do things to make it more difficult to prove any possible case against them. [Note: I'm not saying they were involved]
So I ask myself why would she get involved in trying to determine if the shutters could be opened from the outside? Firstly it would be make sure her children can't be implicated in any way. Secondly it would be to make it seem impossible to do so, even if it wasn't impossible to open from the outside.
I have not been able to work out if Gerry's attempt was before or after Dianne's.