"When religiously-motivated or -justified behaviours are strongly at variance with societal norms, then this will inevitably be a subject for debate. Is it is a legitimate subject for debate?"

Of course it is. If you criticise the behaviour of a persecuted minority, you must tread carefully though.  And you cannot violate their human rights (I know that you don't see bans on headscarves as such, but you will at least concede that your view is controversial, right?) and start a nice unprejudiced talk about their religious praxis at the same time. You will have to decide which you want. With strong Islamophobian movements around you can't have a discourse with Muslims without rejecting the Islamophobian attempts and actively defending the Muslims' rights.

"Random example : in my home country, I remember a controversy concerning Jehovah's Witnesses who refused blood transfusions for their children"

Which is a matter of weighing different, conflicting rights against each other, not a matter of making a minority rightless.