Ok, I'll ask it in big letters this time, so you can't possibly misunderstand what I am asking: Why was it left on the bench?
Well, assuming that the flashlight was brought by an IDI and was wiped clean as you say, then it was probably not deliberately left, but merely forgotten.
Answered already. No crime scene was staged in the wine-cellar by an intruder. The body was left there, period.
Well, theories proposed by RDI such as 'evidence' of the flashlight, can only be described as extremely fanciful. Firstly you say it was 'wiped clean inside and outside', apparently the Rs needed it because they didn't want to turn on lights in their own house, this was purportedly because they were killing their daughter in the basement and didn't want to alert the neighbours. So when I ask why, these clever and devious parents would clean the incriminating flashlight outside and inside (so as to leave no trace), and then leave it on the kitchen bench, rather than replace it in it's usual place, you give no answer.