Hi Fori,
There is some confusion here. Your Pitot heater isn't frozen.
The AIR LO TEMP annunciator means the temperature in the wing and tail anti/de-ice air supply ducts is low. This is usually because of insufficient air-flow.
In your picture your ENG AIR supply switches are off - the APU air is not sufficient for ice protection.
It is usually not required to run the inner-wing de-ice at the same time as the outer wing/tail anti-ice. This is because in the decent through icing conditions the engine air flow is usually not sufficient at lower powers.
When flaps are extended beyond flaps 18 then you would utilize the de-ice, along with use of engine air. Speedbrake is handy in this case for keeping engine speeds up sufficient to provide sufficient bleed-air flow.
Just many of the plates you have to spin/juggle flying a 146/RJ!