I don't think either of those represent a big risk to him. Consider that he knows this particular child will be under Dumbledore's constant care, and that Voldemort himself will always be available to step in and rescue the kid if there's a risk of him dying early. Obviously that doesn't lower the risk to zero, but it's still a big reduction. Also, remember that during the "grooming" phase of this plan it'll be an eleven-year-old against the most intelligent and insidious wizard in history: given what Voldemort did to Bellatrix it seems like he would be very confident of successfully being able to brainwash Harry.

So, the risk of Harry dying or being unbrainwashable seems low, but it also pays to consider that either contingency wouldn't stop the plan in it's tracks. If Harry dies (or has to be killed because he can't be brainwashed) there are a whole battalion of potential replacements who could emerge as the new posterboy for good - Neville being clearly at the top of the list. Harry is the >best< choice, but he isn't the only one.

I've actually cooled a little on this theory since the latest arc began because I think someone is clearly trying to turn Harry evil (see my theory from earlier in the thread). That said, I still think this would be a workable plan for attaining uncontested dark lordship.