La grand-mère mangea le gâteau et but le vin que le Petit Chaperon Rouge avait apportés. Elle s’en trouva toute ragaillardie.

can be melt in one sentence :

La grand-mère se trouva toute ragaillardie d’avoir mangé le gâteau et d’avoir but le vin que le Petit Chaperon Rouge avait apportés.

So, en isn’t a reference to the food but :

a) It refers to the fact of eating and drinking

b) You are right, she is cheered up but because she consumes them, not by just their presence.

c) You can’t say

Elle se trouva toute ragaillardie du gâteau et vin.

Because you cannot ommit “du” (= “de le”).

Grammatically saying the following sentence is right :

Elle se trouva toute ragaillardie du gâteau et du vin.

but is not what we say, we would prefer :

Elle se trouva toute ragaillardie par [le fait de manger] le gâteau et le vin

EDIT : To answer to you comment, the previous expression between brackets can be omitted, because the most often thing done with food and beverage is to consume it.

Using “par” rather than “du” give a movement indicating that the satisfaction effect comes from the food and wine. “par” does not implies that it has been eaten but that her granny has been “ragaillardie” because Le Petit Chaperon Rouge brought cake and wine and they could share it to take back colors on her granny’s face.