p. 240: “Enemies gain +3s to their disposition in arguments and speeches made against you.”
This is presented in the context of new enemies that result from the Enmity Clause in a Circles test. Is this also the case for the Enemy the player creates when they burn up a new guardmouse?
I don’t see why not, an enemy is an enemy…
Makes sense to me, too, but it’s not mentioned elsewhere prior to p. 240. Might be after, but I am still reading the book. 
I would say that it only applies to enemies brought in under the Enmity Clause because this is essentially a twist brought on by the failed roll. You stated enemy on the other hand is created during PC creation and I don’t see why they should get a mechanical advanatage against you in conflicts.
See, that makes sense, too!
For whatever it’s worth I disagree. I would think the +3s most definitely applies to the enemy a player names at character creation. An enemy’s an enemy whether he is brought in at the beginning of play or through the Enmity Clause. But I don’t have anything in the book to back this up; it’s merely my reading of the rule. Your interpretation is certainly supported by the fact that this rule is under the Enmity Clause section. I would just choose to apply it in all cases.