You forgot about the fuel problem. If you stuck in ice, your cruising level is decreed, like in the here mentioned case, from FL190 down to FL130, the fuelplanning is not correct anymore. If not prepared for this you could easily land below minimum diversion fuel. 2 other reasons why fuel planning in ice is delicate. Performance in ice is less than in clean weather, less IAS with same powersetting, more FF for same speed. Early descend is needed and higher speeds/pwr-setting in descend needed. Maybe idle descent not possible because bleed air is not enough for cabin and deicing.
But all airplanes have limitations. The problem starts if you fly your current type like another type. You have to consider actual stall speed in ice could be high as 165 KIAS. (BTW, you can stall any AC at any speed, it depends on G-load). I've seen blackbox printouts from actual stalls of EMB-120 in ice that happens at KIAS 155. In C421 I experienced few at KIAS130. Laminar airfoil have tendency to stall at much higher airspeeeds in ice than clean.
Secondly manufactor recommend Flaps 15 as seen in CL, what indicated Fuselage create to much drag when maintaining higher AoA. (I remember the same for SD3-60, in case of engine failure.) Despite of CL airframe (fuselage) of ATR is not antiiced, or?