Is it economically reasonable to cool the roof by water evaporation
Won't work.
The insulation is sufficient to reduce the heat loss from your apartment at 25c to the snow or ice at -15c to a reasonably low value.

Misters or evaporative coolers only reduce the temperature by a few degrees C, and most of the heat absorbed would come from the surrounding air, not your house.

I'd suggest awnings over the windows, drapes/shades, and an attic fan, and possibly an indoor mister.