Those dust devils may be a few hundred metres tall - but Phoenix will not be under it's parachute at that time. The lander will seperate from the backshell and parachute at an altitiude of around 750 metres.

Even if the decent went thru a dust devil - the attitude control of Phoenix during terminal decent is very powerfull. Phoenix is a fairly compact vehicle before it lands - so the wind ( which would have little force given the atmospheric pressure ) would do little to the lander anyway.

As for chances of success... Phoenix has been reviewed and tested beyond belief ( see the MARDI cancellation for example ) ... I'm sure the engineers would say 90%+

Doug