… manejar la pluma, mientras el entendimiento aterido intenta engañarse,

… manejar la pluma, mientras el entendimiento aterido intenta engañarse, buscando en el regalo …- meaning? …while the frozen understanding??

I’d translate as “numb” in this context. Aterido means “suffering from cold” in general, but it can also refer, metaphorically, to the typical effects of having been exposed to cold temperatures.

But, what then does this phrase mean? - while the numbed mind tries to trick itself?

I’d need a larger context to translate it. Something like:
while the numbed intelligence/mind tries to fool itself,…