You may view it as cowardly not to inform perfectly innocent people of your suspicious views regarding their nearest and dearest, I however regard it as a) good manners not to b) none of my business in the first place and c) preferable not to cause distress to others simply for the sake of airing my "doubts" publicly.
You asked if I would be comfortable telling them of my doubts and I said I would, although not at the venue you suggested unless they raised it first. In fact, my good manners would prevent me from raising the subject at all. It would be cowardly of me if they raised the subject and I agreed with whatever they said, but I would probably only speak up if the alternative was to mislead them.
If it's none of your business why are you here?
I'm pleased that you're so concerned about others, but you are involved in the continuing discussion of the case on the internet. That must, by keeping the discussion going, contribute to any distress caused surely? Perhaps you think your support is noticed and appreciated by the McCann family? I would guess they lump us all in together myself.