If you wish to undermine religious authority that means reducing the number of religious adherents.
These divisive tactics with insults, misrepresentations, and minority oppression are not effective towards those ends.

Islamophobia is playing into our enemies hands; they rule via divide and conquer.
As leftists what we should aim for is mutual understanding and cooperation.

We want religious (and non-religious) people to become more accepting, reasonable, and less reactionary.
Insults only put people into a defensive position. And this only empowers religious authorities.

When people publicly insult Mohamed, where do Muslims turn to for support and solidarity?

You have to be more pragmatic about this. There is no slippery slope when your position is rooted in utilitarian concerns.
If freedom of expression were meaningfully at risk then we could simply pivot our tactics.
Those who insult Islam have done nothing wrong; but those insulting Islam to hurt and marginalize Muslims within society are no better than any other bigot.

Terrorist attacks in response to such expression is already systematically opposed, so what more do you want?
Mosques and Muslim communities are already under surveillance throughout Europe.

Freedom of expression does not mean we have to celebrate every time somebody says something that offends others.
If anything, freedom expression used in this way and on such a scale, causing violence and social unrest, is what puts freedom of expression seriously at risk.

If I thought that via Mohamed cartoons within three years we would have millions more ex-Muslims; then absolutely publish away.
But this same shit has been going on for over a decade and I don't think it's working.