A part of an entry about Past Perfect (424.1) in Swan's Practical English Usage 3rd Ed states that the past perfect is not necessary in the cases where we use conjunctions (e.g. after, as soon as) and are not 'going back' from the time that we are mainly talking about, unless we want to emphasise the separation of two actions. So something like 'After it got dark, we came back inside.' is OK.
Sadly the book's got no example of a question using the past perfect this way. I doubt if we NEED to use the past perfect when we ask questions like 'Had you visited there before that trip ?' Is it also possible to say 'Have you ever visited there before that trip ?'
And for 'Had you read the instructions before you turned the device on ?' Does 'Did you read the instructions before you turned the device on ?' sound good ?
Are there any differences in the meanings of them ?
My understanding now is that they all are not wrong but when using the past simple form, we just don't emphasise that the actions are separated, one finished before another.