She assumes that evil is obvious.

I think you are ascribing positions to the narrator which she makes pains to hedge against. She is certainly providing the structure of the argument with almost no elaboration regarding the arguments against it, but she points out in many cases where those arguments are often levied.

You are suggesting two common arguments against this argument, although they aren't compatible arguments.

God is the universal standard of what good is.

The first argument is the claim that there is no evil. This is an argument that God is the definition of good and thus anything which is allowed in the universe is good. This runs into the problem that most people when they assert the omni-benevolence of God are appealing to some knowable human notion of 'good' which generally doesn't include things like horrible suffering for innocent children. It basically says that we don't know what metaphysical Good and Evil are so God can have this property even if we are (incorrectly) unhappy with the results.

Just because God could prevent evil doesn't mean God should prevent evil.

This is the argument that evil does exist, but it exists to achieve the greatest possible good. In order for this argument to stand, one must find a way to come to this conclusion by tweaking the premises.

The easiest way of doing this is to suggest a logical limitation to omnipotence. We accept that certain things are logically impossible (e.g. a boulder with the quality that God can't lift it). If God's omnipotence is limited by logic, it is possible that the evil which does exists must be logically necessary to achieve the greatest good. You even bring up the idea that for some reason, 'free will' is part of the greatest good and that is logically incompatible with no opportunity for evil.

This runs into the problem that it isn't clear that the goodness of this degree of free will is rational in relation to the great amount of evil which is permitted in its pursuit; one could easily conceive of just a bit less free will where there is a great deal less evil. This once again brings us to questions of the definitions of good and evil and how much people should be swayed by an alien definition of good which seems at odds with the obvious preferences of humanity.