I agree with Paul. The "vol" in "omnibene
volence"
means will (and in fact has the same Indo-European root as our English word "will"). So it means that God's
will is to do good.
To will one thing rather than another cannot be seen as a
constraint on one's freedom of will without rendering the concept of "free will" meaningless --- no-one's will can be so free as to be free
of their will.
Further, you write:
Our only demand for an omnipotent being is that it COULD do anything and everything.
Now I think that the ambiguity of this has mislead you. For there are two ways of reading "COULD do anything and everything":
(1) Could, in principle, do whatever it wants.
(2) Has an actual possibility of doing something that it doesn't want to do.
Case (1) is omnipotence. Case (2) would actually be contrary to omnipotence --- a being that does something that it doesn't want to do would
not be omnipotent. It would have the divine equivalent of Tourette's syndrome, being unable to control even its own actions.