it's become apparent to me through reports of various cases that the police (in various countries) don't always preserve the crime scene in the way they should.
I agree with this. There's a whole heap of surprises to us, particularly those of us in the UK.
There, all the witnesses called security before they called the police. That's apparently what they do there. We just pick up the phone and go for 101 or 999.
The ambulances were odd too. Pistorius seems to have called one company, and the estate manager another. Oscar was (apparently) told to move the injured woman in a way that makes no sense to us. The estate manager had a long conversation, needing to hand over to his daughter because the ambulance were trying to decide where it was and how to get there. Again, this makes no sense to us with our knowledge of the UK service.
It doesn't seem odd to the people in the courtroom that OP would have a houseboy, nor that he would refuse to talk. This is just accepted. Here, if we had, say, a cleaner, on the premises when a crime took place of course she or he would be questioned. I for one would like to know whether OP instructed Frank to make sure the ladders were removed at night. But Frank is pretty much a non entity.
The cultures are massively different in so many ways.