As BEA puts it on page 30 of Interim Report No.1:
the presence of super cooled water at FL350 is not very probable and would necessarily have been limited to small quantities.
IMHO the heating of the pitots is designed and has been demonstrated to function in any quantity of supercooled liquid water likely to be encountered. The icing detectors would have detected the presence of significant quantities of liquid water.
Clandestino,
For distribution of liquid water and ice particles in convective clouds, read IR#3 page 50.