1- From experience, SCH can pull the most shenanigans. SCH was able to solo T5 when 2 WHMs struggled to heal it before gear trivialized it for example. SCH got a few "sneak fairy healing" nerfs but I still think it's a stronger healer than the others.

2- AST is a bit lacking. But none of the healers "Screws" you.

3- Pre-3.0 statistics said WHM was the most played class of all. Assuming that, WHM is most played. There are more scholars than Atrologians because AST is relatively new and groups favor WHM/SCH combo.