In English, I use the phrase "to play Devil's advocate" quite often, it allows me to ask difficult questions without challenging the other person's position or status too directly.
The nicest translation I have found so far is this one (from Linguee):
Mal ein bisschen ketzerisch sein
So let's play Devil's advocate for a moment and ask who might buy this fountain pen and in particular, why would one want to buy the latest product from Lamy?
Sind wir mal ein bisschen ketzerisch und fragen: Wer könnte sich diesen Füllfederhalter kaufen, und, vor allem, warum sollte man sich dieses neueste Produkt aus dem Hause Lamy zulegen wollen?
As you can see, it allows us to be direct and maybe a bit cheeky, but still not be offensive.
EDIT - to clarify the question in response to feedback - I am looking for a phrase which is used commonly in everyday German, not necessarily a direct verbatim translation of the phrase "Devil's Advocate".