You will have to be lucky, I would say the chances of any lifts being
open is about 50/50 but if they are open there will be a few runs with
a thin cover. I havn't skied Nevis range but Aviemore is about as
high.

just to be optimistic I have skied both Nevis and Aviemore on the 13th
and 14th of May in 1994 (pics here) with good coverage.

http://www.gregh.co.uk/html/ski.htm

I've also skied Nevis on the 21st April 2001 with good coverage and
great skiing in the Back Corries, as well as the 8th April 2000.

Having said that the past few years I think all skiing has been finished
either before or just into April.

regards,

greg