Really, the only viable FWA for this route is as Mike describes. In moderate rain or wind, you'd get away with following the path around Carn an t-Sagairt Mor then descending Allt an Dubh Loch but, given that this rises to over 3000 feet, it's hardly a place you'd want to be if conditions were extreme.
Don't get too concerned though, as I can think of only a handful of occasions in the last twenty years when folk have had to retreat from Callater. It's rare for the weather to be that bad in May when you're so far east.

Colin

All I would add to Colin's comment is that you check the weather forecast before leaving Braemar. If it looks doubtful stay on the camp site overnight and make your decision in the morning. Using either route you should be able to reach the Spittal of Glen Muick in one walking day.