The Euthyphro dilemma ricochets

I. A favorite atheist objection to Christianity in general, and the moral argument for God in particular, is the Euthyphro dilemma. For a brief exposition:

i) Is an action right (merely) because God wills it? 

Or

ii) Does God will an action because it is right?

As formulated, if the Christian (or theist) opts for the first horn of the dilemma, then that seems to make morality an arbitrary divine fiat. 

Conversely, if the Christian (or theist) opts for the second horn of the dilemma, then that seems to make morality independent of God by grounding morality in a higher standard, apart from and above God. In that case, God's will is superfluous to ground morality.

II. Now, I've addressed the Euthyphro dilemma on many occasions, so I won't repeat myself here. Instead, I'd like to flip the objection. For it's easy to generate Euthyphro dilemmas for secular ethics. For instance:

1. Evolutionary ethics

Is an action right because we're hardwired to deem it right, or does the rightness of the action determine the rightness of our hardwiring? 

Same conundrum for Neo-Aristotelian naturalism. 

2. Contractarianism

Is an action right because the social contract makes it right, or does the rightness of the action determine the rightness of the social contract?

3. Consequentialism

Is an action right because the consequences make it right, or does the rightness of the action determine the rightness of the consequences? 

III. Finally, atheists object that grounding morality is God is an arbitrary stopping point. But isn't the objection reversible? Why isn't grounding morality in evolutionary psychology, consequences, or the social contract an arbitrary stopping-point?